Edward:
Check out this marine site. They have lots of various info that relate
to what your interested in. On my boat I have a starter battery and
another set of house batteries. When running a charge in from either the
engine's alternator, the solar panels or the battery charger if I'm tied
in to shore power - it all goes the same way - to the main house bank
first, from there the current bleeds over to the single starter battery
to keep it charged. Between the house bank and the starter battery is a
solenoid. This is activated by the incoming charge. When that goes away
the solenoid breaks the connection so that that the connection is
broken. This prevents the house bank for draining the starter battery if
it should malfunction for some reason. (it better not) You want to do
the same thing in your van.

http://www.amplepower.com/

Jim
90 GL (still at the shop)(should have done the work myself!)