Since there always seems to be interest in auxiliary battery installations, I thought I'd report on mine.
The installation in my '88 Westfalia included some major accessories and some re-wiring, as well. I think the most unique feature of the installation is the location: everything (including the battery) is installed in the "hidden compartment" in the driver's side cabinetry. That's the compartment that's reached via the "trap door" in the floor of the storage bin that's just aft of the refrigerator. Normally, it houses just the 110V outlet and circuit breaker. Since its inaccessibility makes it not terribly useful as storage, it seemed the perfect spot to use. (In contrast, I've found the more typical auxiliary battery location - the bin behind the driver's seat - useful for storing the camper's curtains and screens, and I didn't want to give it up.)
The major components installed are as follows:
Douglas size U1 31 amp hour sealed gel cell. Size U1 is just right for the hidden compartment. 31 amp-hours is about half the capacity of the popular Optima battery that's usually put in the bin behind the driver's seat, but it's plenty for us. Plus, if necessary, we can draw a little from the starting battery, too (just as we did before adding the auxiliary battery).
Battery charger / power supply
Guest Battery Pal Magnum 3 amp charger. Charges the battery and runs 12V appliances when the camper's plugged in to 110V. Very small - about a 4 inch cube. This is plugged into a 110V outlet I added inside the hidden compartment, so it supplies power automatically when we plug the van in.
Whistler PP140AC 140 watt invertor. This is plugged into a switched 12V outlet I added inside the hidden compartment, and half of the camper's standard 110V interior outlet is plugged into the invertor. (I "split" the outlet; the other half is still on camper's standard 110V system). That gives me one "shore tie" outlet (for when the van's plugged in), and one "invertor" outlet that I can power any time by switching on the 12V outlet the invertor's plugged into. (I chose the Whistler because it handled this switching of the 12V outlet without complaining. The Vector invertors I tried sounded a "power fault" alarm when I did that.) The switch is installed in the original "circuit breaker" panel below the interior 110V outlet (along with the circuit breaker).
Tekonsha 100-amp 4-terminal battery relay. This sits between the new battery and the starting battery, and is wired to the original "alternator running" signal wires that run to the original refrigerator relay (behind the driver's seat). When the alternator's running, this connects the two batteries together, so both charge. When it's not, the batteries are disconnected, so I don't accidentally drain the starting battery by running the 12V appliances. I also added a "manual connect" switch that bypasses the relay and connects the batteries. It can be reached through the small "notch" in the left rear of the floor of the bin above the hidden compartment. (I have no idea why that notch was originally there, but it works out great for the switch.) "Manual connect" is useful for three circumstances: (1) To draw power from the starting battery, if we've drained the auxiliary battery (hasn't happened yet). (2) To charge the starting battery from the battery charger (or from the auxiliary battery itself). (3) To jump-start the engine, if the starting battery's dead.
I've got a 50 amp circuit breaker between the relay and the starting battery. That's fine most of the time, but if I jump-start the engine from the auxiliary battery, the breaker will trip after about 5 seconds of cranking. (It resets itself after about a minute). That's probably adequate, but still, I'll probably make myself a little foot-long "jumper cable" to bypass the relay, breaker, and manual connect switch (which is only rated at 50 amps) for jump starting.
New 12V interior outlet
West Marine 12V outlet, installed in the little panel with the circuit breaker and invertor switch. Yes, that panel is now pretty crowded, but not unreasonably so. I installed a new, bigger electrical box behind it, and drilled 3 holes in a new "blank switchplate" panel for the switch, outlet, and circuit breaker. It's very convenient to have a 12V outlet back in the living area of the van. Bedsides, the original dash "cigarette lighter" outlet is very shallow, and most 12V plugs I've tried make poor contact and fall out of it.
New light over rear seat
Hella Adjustable Courtesy Berth Light. Installed in the center of the face of the air conditioner housing, above the rear seat, between the two speakers. This light has a little "cover" over it; roll the cover back, and it switches the light on. You can (somewhat) adjust how bright the light is by how far you roll back the cover. It's bright enough to read by, but generally doesn't bother the driver at night. I also installed a second switch for the light in the headliner over the driver's seat, right by the map light. This is very, very useful for seeing how the kids are doing when we're driving at night, or for checking out "what made that noise back there - did something fall over?", etc. By far our most-used new accessory.
Auxiliary fuse panel
Uses automotive blade fuses. Has unfused connections for the auxiliary battery, charger, and pump/fridge (which retain their original fuses behind the driver's seat). Has fused connections for the 12V invertor outlet, the new 12V interior outlet, the lighting/furnace circuit (which includes the original cabin light, the new rear seat light, the Propex furnace, and a few wires leading various places for future use), and the original auto accessories circuit (which includes the radio, map lights, and dash 12V outlet).
The major wiring change (besides the wires for the new stuff) is that the auto accessories circuit, the fridge, and the pump now run off the auxiliary battery, rather than the starting battery.
That's pretty much the summary. More detail follows, for those who care.
Disclaimer: I'm not an electrician, nor particularly experienced with rewiring cars. This installation has worked well for me, but I may well have done something stupid and dangerous that I just haven't discovered yet.
8-gauge wire leads from the battery's positive terminal to the battery relay, then through the 50-amp breaker, then down through the floor, across the underside of the van (slipping between the frame and the floor) to the big "terminal 30" of the starter solenoid. Terminal 30 is connected directly to the starting battery via a very large gauge cable, so this is a convenient place to tie in. The hole in the hidden compartment's floor is sealed with liquid electrical tape. This is one of only two holes I had to drill for any of the wiring.
The battery's negative terminal is tied to the original "body ground" inside the hidden compartment, that's used for the original 110V outlet. This ground point is a clamp that holds the green 110V ground wire. I replaced the clamp's setscrew with a bolt (some strange metric size; wish I remembered what it was), added a ring connector to the 110V ground wire, and used the bolt (with a lock washer) to attach the battery ground and the 110V ground to the ground point. The battery ground is a short 4-gauge auto or lawnmower insulated ground strap. Note that the green 110V ground wire grounds the circuit breaker's metal electrical box; I attached a "multiple quick-disconnect" tap (from Radio Shack) to the side of the box, to use for anything else in the compartment that needed grounding.
Cabinet wiring conduit
I needed to run wires from the hidden compartment to the bin behind the driver's seat, so I ran flexible plastic split-tube conduit along the floor against the left wall from the hidden compartment, through the cupboard just forward of the hidden compartment, under the fridge, and into the under-sink cupboard, then up the wall and out behind the driver's seat, and into the driver's seat box. There were already holes and notches in all the right places in the cabinetry - I didn't need to cut any. I removed the "false floor" in the under-sink cupboard so I could run the conduit under it. (I may leave it off - I can't see much reason to have it, and it just decreases storage space). The only inaccessible spot is under the fridge, and it was easy enough to push the conduit through from one side and catch it on the other, with a little help from a stick poked under from the front of the fridge. The conduit was attached with screw-on wire clips where needed. The wires were already in the conduit when I ran it - I don't think they could be pushed through the conduit after it's in place.
The wires in the cabinet wiring conduit are:
Auto accessories power
Runs from a fused circuit in the auxiliary fuse panel, through the cabinet wiring conduit, to behind the driver's seat, then under the carpet to behind the dash, and up into the fuse panel. I removed the "accessories" fuse, and clipped the new wire into the "load" side of the fuse socket. I used a standard male quick-disconnect, but I think there are special taps made just for this, too. The fuse socket remains empty - if a fuse were put there, it would directly connect the auxiliary battery to the starting battery (through a small fuse and thin wires), which would be bad.
Runs from a fused circuit on the auxiliary fuse panel, through the cabinet wiring conduit, to behind the driver's seat. There, it becomes the positive half of a 16-gauge 2-wire zip cord. The negative comes from the bin behind the driver's seat, where it is bolted to a convenient threaded bolt hole on the front "ceiling" of the bin, to ground it. The zip cord runs through a new hole (the second of the two I had to drill) into the pillar behind the driver's door, up the pillar, into the interior of the curtain rail/shelf above the cabinetry. You have to remove the curtain rail/shelf for this - the procedure is detailed in the Bentley manual. From there, it splits into more 2-wire zip cords:
This is actually more complex than necessary, and I ended up doing it this way for historical reasons. Simpler would be to leave the cabin light power as-is, since the auto accessories circuit has been switched over to the auxiliary battery anyway. The new rear seat light could be powered from the cabin light, but I wouldn't want to add much more than that, since the wires from the driver's map light to the cabin light are pretty thin. For the furnace, a separate wire could easily be run directly from the hidden compartment to under the rear seat. Unless you wanted the rear seat light, there would be no need to take down the curtain rail/shelf.
Runs from an unfused circuit on the auxiliary fuse panel, through the cabinet wiring conduit, to the bin behind the driver's seat, where it replaces the original power wire on the fridge relay.
"Alternator running" signal
Runs from the two "alternator" connectors on the fridge relay, through the cabinet wiring conduit, to the new battery relay.
Rear seat light
The rear seat light has three terminals - "common", "external switch", and "internal cover switch". The power from the lighting/furnace circuit goes to "common", and the ground to "cover switch". The "external switch" wire follows the lighting/furnace circuit wires into the curtain rail/shelf, and then into the headliner above the driver. There, it goes through a pushbutton switch and an automotive blade fuse in a holder (to protect the switch), and then to the ground terminal on the driver's map light. The pushbutton switch pokes through the headliner just aft of the map light, and the fuse holder and wires are stuffed up there behind it. The wiring can be done just by removing the map light - there's no need to remove the headliner.
The 110V outlet was "split" (electrically) into two separate outlets by breaking the two metal tabs provided for that purpose on the sides of the outlet. (Those are typically used in house wiring to connect one outlet to a switched circuit, and the other to an unswitched circuit). I think I actually replaced the outlet with a new one that had more places to attach wires, but this isn't strictly necessary. One of the outlets remains wired as before. For the other, I cut about a foot off of the male end of a a heavy-duty 3-wire extension cord, and wired it into the outlet. The other end is then plugged into the invertor. (Actually, a 2-wire cord would probably do the trick - the "ground" socket in the invertor's 3-prong outlet is a fake, and isn't connected to anything in the invertor.)
I cut about a foot off of the female end of the same extension cord I cut up for the invertor, and wired it into the non-invertor ("shore tie") outlet. The battery charger plugs into this extension cord.
The battery sits in the aft outer corner of the hidden compartment, atop a little styrofoam platform. The platform has a channel carved in the bottom for the original 110V wiring on the floor of the compartment. I carved the channel using a knife, a small saw, and the flame from a lighter (to make it smooth and "sealed"). The terminals are on the forward side of the top.
To get the battery in, it's easiest to remove the wall between the hidden compartment and the storage compartment that's just forward of it. It only takes 5 screws - only one more than getting to the starting battery!
The battery relay and circuit breaker are attached to the aft wall of the hidden compartment. The "manual connect" switch is in the aft outer corner; I put mine in a little plastic box so I could stick it to the wall easier.
The charger attaches to the aft wall of the compartment, over near the 110V outlet's electrical box.
The 12V outlet for the invertor is on the floor of the compartment. The invertor itself is on the forward wall. It's hooked securely, but not permanently, onto two screws in the wall. That way, it's easy to take off when it's time to remove the wall to replace the battery (presumably not for many years).
The auxiliary fuse panel is screwed to the underside of the unmovable portion of the floor of the bin above the hidden compartment (that is, the compartment's "ceiling"). The fuses face toward the passenger side, and are easy to get to.