Roberts manual switch is a good solution and all his points make
sense....but it you want to have an automatic solution, my vote is
for the Suerpower 1315 separator (especially with Subie engines)

There are many ways to solve dual battery problems in a Vanagon.
Each owner has to decide what is the best solution for them....just
like tires!

BTW, what is your favorite tire for a Vanagon?  (just kidding)