Stancor #120-901, heavy duty DC power contactor, 100 amps continuous,
400 amps surge, $15

I have hooked up numerous second batteries for Vanagons and other VW
campers. I prefer to use a relay as this does not require any cutting or
splicing of the original vehicle wiring. It can thus be easily unhooked
for trouble shooting or whatever.

The other approaches have their merits but I like to leave the original
wiring alone as much as possible.

I have used various relays and switches but my favorite is this one,
available from Newark Electronics. I used my last one recently and I
ordered 5 more today.

http://www.newark.com/     Newark part# 50F3662

These can be securely mounted in the battery compartment next to the
battery and all connections are attached by nuts. I like at least 8
gauge Marine grade wire between the batteries for camping use but
heavier gauge is needed if you also want to use the aux battery to
assist during starting. This capability will depend on what you arrange
for controlling the relay. In most Westys I use the fridge relay output
to trigger the aux bat relay and this means the aux battery is NOT
connected to the main battery during starting. The batteries are
connected together only when the alternator is charging and the
switching is done automatically.

This also allows a simple way to use both batteries in a pinch for
starting. By removing the small fuse cover behind the driver's seat and
bridging the 2 fuse holders with a coin or such the aux battery relay
will energize and connect both batteries even with the engine off.

Mark