RE: Aux Battery Install
Go here:
http://www.geocities.com/harald_nancy/dualbattery.htm
Virtually all you need to know on installation (but doesn't cover the 1315)
Stock batteries are BCI Group 41.  Although I've never used them I
understand that a Group 91 will also fit and has higher capacity.  There are
also instructions/wiring diagrams in the Bentley manual.  With standard aux
battery install the interior accessories (lights and radio) will still be on
the starter battery.  A simple wiring change can move them to the aux
battery, exact wiring depends on the model year of your Vanagon.  There are
no full size deep cycle batteries (Group 41) on the market.  Many people use
the Optima Yellow Top batteries by modifying the battery compartment so the
Optima will fit.