It is not advisable to use a dual battery isolator with you VW van. This is
because isolators typically reduce the voltage by about 1 volt. On an
American car, which might have an alternator which puts out 15 volts plus,
this is OK because you still get 14 volts after the isolator to charge your
batteries. But VW designs its alternators to put out fewer volts--I cannot
off the top of my head remember the amount--but lets say it is 14. After the
voltage drop from the isolator, you are left with only 13 volts, not enough
to recharge your batteries fast enough. 

This characteristic of your VW is not a defect. VW simply felt that 15  volts
was too much and would overcharge your batteries. The USA car makers don't
feel this way.

The solution is to use relays. There are 200/500 amp relays that you can buy
and connect up to disconnect your batteries automatically for various
scenerios. Wrangler Power products of, I think, Scottsdale, AZ has a great
catalog with various wiring accessories for this purpose.

Think twice before using an isolator in a VW!