I have my aux battery under the passenger seat in a homemade box.
Those meters are for checking the aux & main battery voltage:

http://members.cybertrails.com/~ahwahnee/VWBattery1.JPG

Advantage, of course, is that one can use a standard size deep cycle
battery -- pretty cheap from Wally-Mart and such.  Wiring is simple
enough if you're comfortable with electrics though mine does look a
bit like a bird's nest I suppose:

http://members.cybertrails.com/~ahwahnee/VWBattery2.JPG

There is a power outlet (aka cigarette lighter socket) on the side of
the box for hooking up the inverter or 12VDC stuff.

With the recently added 15W solar panel (<$40 at Harbor Freight) and
Southwest sunshine we never run out of juice even with DVDs and CDs
into the night.

http://members.cybertrails.com/~ahwahnee/VWSolarPanel.JPG

To power the radio, CD, clock, front lighter & front lights no
addition wiring was run forward of the light in front of the closet.
From that point forward the existing harness was used (sort of in
reverse) and inline fuses replaced the 2 removed fuses in the original
fuse block.

I do carry a 4' home-made jumper cable so I can use the + post on the
aux to give the main battery a jump should it ever be needed.

Geo