You guys are driving me crazy talking about this stuff so I have to answer
even though I have done so in the past: 

1. Do not use a dual battery isolator (robs too much voltage from a VW
alternator); use electrically operated relays instead. 

2. You can fit a deep cycle marine battery behind/underneath the driver (of a
Vanagon) if you allow the battery to stick up slightly higher than normal and
then fabricate a rug/tin covering to hide it. If you have swivel seats then
trim away the right rear corner of the swivel base so you can still swivel you
r seat without hitting the battery. 

3. Do not ever wire your batteries together since the one will drain the
other; always use relays to keep at least one of them disconnected when the
motor is off.

--Derek Drew

P.S. My new address is going to be: drew@interport.net although I will retain
my old aol.com address indefinitely.