I installed a Carver P4 propane furnace in my camper in fall of 1999. Here's my report on the installation. It's pretty long, since I've tried to give enough detail to help anyone else who might be attempting a similar installation.
I installed the furnace under the rear seat, to the right of the existing heater under the seat. (That is, to the right if you're facing forward - thus, toward the passenger side. I'll use that convention for "right" and "left" throughout the text below.)
The furnace is mounted horizontally, on the floor, with the air intake and outlet to the left. There's just barely enough room to fit it in (once the ducts and flues and such are taken into account). In order to get the furnace rearward as far as possible - and get as much room for the exhaust flue and combustion air intake as possible - I reversed the 90 degree bend in the two rear mounting brackets. (Used vice grips, pliers, a vice, and a hammer. It went poorly, and took a long time, but I eventually got what I needed.) Normally, they bend away from the furnace; now they tuck under it. That allowed me to get the furnace closer to the sloping rear wall of the compartment. Since there's a fairly large gap between the wooden floor and the rear wall, I also drilled new mounting holes in the forward edges of the brackets, so that I could let the back edge of the brackets extend rearward of the wooden floor. (If I had it to do again, I think I'd also try to fill in that gap somehow - then, I could move the furnace perhaps another half inch rearward, which would be a help in putting in the flues.)
This left just barely enough room to bend the two flues in a pretty sharp 90 degree bend down into the floor. (It's a lot less room than the Carver manual wants, though.) The holes were drilled just big enough for the flues. Somewhat-too-small hole saws were used (since the exact sizes needed weren't available), and the holes were enlarged as needed using a rotary rasp on a drill. The front edges of the holes are about 1/8" from the front wall of the compartment. (The Carver manual templates show holes that are significantly larger than the flues. Based on my conversation with Carver Tech. Support, I believe this is for ease of installation, and extra protection for floors that are made of meltable materials. The flues don't get hot enough to burn wood, so I believe a tighter hole is OK in the VW. Besides, I didn't have room for bigger holes.)
At the 90 degree bend, the two flues actually touch the front wall of the compartment. This is probably harmless, but since the exhaust flue (or, more precisely, its shield) can get pretty hot, I didn't like it. So, I made a little "heat shield" (about 4" x 8") by wrapping a piece of fiberglass from a furnace filter with a lot of layers of aluminum foil, and tucking that between the flues and the wall. I don't know if it does anything except make me feel better, but it at least does that.
Under the van, the exhaust flue has a short run to just forward of the rear jack point, facing to the right. There's a little metal lip that hangs down there, and I was afraid that, if the flue was just behind it (that is, left of it) it would tend to catch some of the exhaust gases and send them under the van. To avoid that, I put a little wooden block between the end of the flue and the underside of the van, to get the flue below the lip. The corrugated shield around the exhaust flue ends about 1/4" below the underside of the van. I stuffed the shield with fiberglass from a furnace filter, to help keep the exhaust flue centered in it, and to keep out bugs and such. (Probably unnecessary since there's no opening to the inside of the van from the shield, but it made me feel better.)
I wasn't quite sure what to do with the intake flue. I could have sent it forward to underneath the sliding door, but I was hoping to mount a storage box under there some day, and the flue would be in the way. I finally decided that the van sat high enough that the air underneath it would almost certainly be fresh, so I ran it just a bit leftward and rearward to the van frame rail, and then bent it in a U so it faced to the right. (The Carver manual said to keep the two flues the same length, and pointing the same direction, presumably to avoid creating an unwanted draft through the combustion chamber. However, Carver Tech. Support told me that having the flues be different lengths wouldn't really be a problem. I decided to play it safe and still keep them facing the same direction, though.)
The propane line comes out of the furnace, turns 90 degrees rearward, and then takes a big horizontal U bend, which has a valve in it to shut off the gas in the summer (or if the furnace should need service). It then dives through the floor, and goes under the van (between the frame rails and the underside) to the propane tank. There, it's tied into the "stove" line with a T. (It probably makes no difference, but I chose the "stove" line because I figured the fridge was the more trouble-prone of the two existing gas appliances, so I was less inclined to mess with its gas supply line.) I made sure to leave enough room to still be able to remove the propane tank if necessary.
I put a bead of silicone around the two flues and the propane line, on the underside of the van (after cleaning the area with alcohol). I didn't use a "high temp" silicone - the temperature rating on some standard stuff I had lying around was plenty high enough.
I attached about 9" of duct to the blower outlet. It takes a very sharp 90 degree turn (there's not much room between the furnace and the existing heater box) through a hole in the compartment front, and is capped with the included duct outlet. I picked the one with the damper in it, though I can't think of any reason to close the damper. I'm thinking of replacing the damper with a little plactic grating, to keep small objects from accidentally rolling down the open duct and into the furnace. I had considered trying to somehow run the duct into the existing heater's short, rectangular duct, to avoid putting a new hole in the camper's "furniture", but decided that was way more trouble than it was worth.
I attached about 3" of duct to the intake - just enough to attach the duct inlet end to it (so no duct shows - it just connects the furnace to the plastic inlet end). Then, I stuck a piece of furnace filter in the duct inlet, to keep dust out of the furnace. (The furnace is used infrequently enough that I think it would be a long time before I'd ever have to replace the filter.) Since the van's existing under-seat heater draws its air from directly under the seat, I figured that would be plenty of air for the Carver, too. There seem to be plenty of gaps around the seat to let air from the van's interior get under the seat.
Power is via a two-wire 16-gauge zip cord (like extension cord wire) that runs to the left side of the compartment, through a corner of the water tank compartment, into the closet, up the left closet wall, out the closet vent (by the window), and into the overhead curtain rail/tray. There, it taps into the cabin light's power and ground, which I have tied into my auxilliary battery. Without the rewiring I did for the auxilliary battery, I don't know if this would be a good idea or not - the cabin light is normally powered off the little dome light above the driver, and the wires there are pretty thin. The P4 doesn't draw much, though, so maybe it's OK. Another option would be to somehow get the wires from the P4 to the fridge cabinet, and draw power from there. At the P4 end, the powered half of the zip cord connects to the P4 fuse holder, and the ground half to a quick-disconnect connector. The wires included with the P4 connect to the other side of the fuse holder, and the quick-disconnect.
The thermostat cable runs along basically the same route, but instead of going into the curtain rail/tray, it goes just above it, and then out between the tray and the plastic air conditioner vent coweling, just in front of the closet wall. The thermostat is tucked into the corner formed by the closet, the vent coweling, and the tray. I mounted it on a square plate of 1/4" thick wood. The plate has a square cut out in the middle for the thermostat electronics, and a slot running from the square to the upper corner by the curtain rail tray. The cable runs through the slot. The plate is held to the wall with heavy-duty double-stick tape. The thermostat is screwed to the plate with short wood screws. The cable is pretty much unnoticable. I also had to carve out a deep round depression in the back of the plate to fit over the carriage bolt head that's in the closet wall in that area. By using the plate, I avoided having to cut into the closet wall at all. Although only a little of the wood plate shows once the thermostat is on it, I put a coat of clear wood finish on it, because it seemed like the right thing to do.
Note that getting to the cabin light wires requires removing the curtain rail/tray, as described in the Bentley manual. The only other hard part in the wiring is getting the power wires and thermostat cable from the closet, to under the seat. Because of the shape of the closet floor, it's fairly easy to snake a wire under the closet's front wall and into the (inaccessible) water tank compartment. If the wire's stiff enough, and you jiggle and push persistantly enough, it's possible to snake the wire through the corner of the compartment and out into the gap in the left wall at the rear of the under-seat compartment. Or, it may be easier to go the other direction - from the under-seat compartment to the closet. I forget which finally worked for me. What turned out to be very difficult was getting the thermostat cable through - the plastic connector is a bit too big to push through. If you have spare connectors and the proper crimping tool, you could just cut off the end, run the cable, and put on a new end. I didn't, so I ended up unscrewing most of the bolts and screws that hold the closet in, prying the front closet wall up just a bit with a pry bar in the closet, and running the the cable (with connector) through the enlarged gap under the wall. I used duct tape to wrap the connector so it wouldn't snag, and taped it to the already-run power wire, which I used to pull it through. Once the connector's through, the closet wall can be let back into its normal position and reattached.
I put a shelf above the furnace, so I could retain as much storage space as possible under the seat. I mounted a couple of small angle brackets on the front wall of the under-seat compartment, with their tops about 3/4" above the furnace. I cut a piece of 3/8" plywood so that it just about filled the area above the furnace. It rests on the angle brackets, and the slope of the back wall. I had to cut a notch out of the right-rear corner, to fit around a protrusion in the wall there. I cut the board slightly narrow, to give a little extra air flow into the furnace. The shelf ends up being about half an inch or so lower than the top of the box around the existing under-seat heater.
For part of the work, I got help from a local RV service place (Rec Tec, in St. Louis). They were great - they were willing to do pretty much any custom work I wanted, on a time-and-materials basis. They did all the work on the propane line, and drilled (and enlarged as necessary) the three big holes for the flues and the warm air outlet. I had them do the propane because I didn't have the proper tools, I didn't feel like learning to flare and bend copper tubing, and because I was more comfortable having a professional work with the exploding stuff. Besides, it struck me as one of those jobs that sounds simple in theory, but that I'd probably end up spending about 10 hours on. I had them do the holes because I figured that wouldn't cost much more than investing in big hole saws I'd probably never use again.
According to the RV service place, the propane fittings for the P4 are pretty unusual. The guy they originally assigned to the project didn't know what to do with them, so they assigned someone more experienced instead. He said the type of fitting Carver was using was more commonly used for water, rather than gas. He ended up needing copper line that's thinner than what's normally used for propane, and had to go buy it. Apparently it's the size normally used for ice makers. We still ended up with one of Carver's fittings left over, which we couldn't figure out what to do with. (That's besides the little metric compression fitting that Carver included a US replacment for - we knew that one would be left over.) Nothing leaks, though, and everything seemed to make sense, so I guess it's OK.
Before I brought in the van, I had everything measured, had the holes drawn, and wrote up instructions. I ended up sticking around and helping while they did the work, though, and I was glad I did, since a lot of questions came up during the work, and I was able to make sure everything was the way I wanted it. It ended up costing quite a bit more than I'd hoped - $174 for 3 hours of work, and $38 for the propane line, valve, and fittings. I think if the same place were to do the same work again, they could probably do it in about half the time - a lot of the time was spent just figuring things out. I guess that's the price for doing something custom.
Oh yes, and I also had a hardware store cut my shelf to size (I'd bought the wood from their scrap pile), since they do that for free, and my little hand saw and I have trouble making straight lines.
I'd guess that I put about 10 hours of my own time into the installation, not counting the time I spent at the RV place, or the time I spent staring around the van thinking "how am I going to install this thing?" (which was more hours than I'd care to admit).
I should mention that I have a propane detector, a carbon monoxide detector, and a smoke detector in the van. I didn't install them specifically because of the furnace, but they seem like a good idea to have.
We haven't actually gone camping with the heater yet, but it seems to do a good job heating the van. On a 45 degree day with the pop top down, it can pretty quickly bring the van up to "way too hot". Last night, I tested it with the pop top up and the upper bed unfolded. It was 45 degrees outside. Running constantly on "high", the furnace was able to keep the cabin at about 69 degrees, but the upper bed area never got warmer than about 52 degrees. I was hoping it wouldn't have to work quite so hard to maintain those kinds of temperatures, but I guess it's hard to heat a tent.
So far, we're very happy with the furnace.