While there are proponents of connecting batteries in parallel on this list
(and elsewhere), it's still not a real good idea due to potential technical
issues*.

UNLESS you need the added current capability - if so, there's no other
choice (other than fitting in a bigger battery).  It's been pointed out that
(electric) submarines and heavy starting applications (e.g., big diesel
engines in cold weather) use parallel connections because they must have the
high current.

But in the Vanagon application, I think the typical reason for having a
separate "house" battery is to provide peace of mind regarding the starting
battery's condition, not a direct need for heavier current capacity.

Then the relay type separator is a good choice.  Solid state isolators
(using silicon diodes) are apparently not such a good choice for reasons
discussed here recently.  Too much power lost in the diodes as heat.

If there are questions or comments, please use p-mail.

//Jack

* A multi-plate lead acid cell can be said to be an application of parallel
connected batteries; ie, the plates are connected in parallel.  But in this
case, the plates are in a common environment, have the same age & wear, etc.
 Individual differences from one plate to another aren't apt to be
significant enough to cause problems within the normal lifetime of the
battery.  With external batteries, this is not so easy.  Yes, I know people
do it everyday.  I'm done.