Dave, the Vanagons that came with an aux battery have an stock wire
running from the main battery compartment to the aux battery compartment
that is used for charging the second battery via a relay. That wire is
stock for dual battery vans. Some people with campers try to add a
second battery without adding this wire that theirs didn't come with.
Instead they try to use the stock wire under their driver seat that
comes from the fuse panel and which is there to provide power for the
camper kitchen and camper 3 way fridge to run on 12 volts. The first
stock wire works fine for its intended purpose of charging the battery.
The second stock wire works fine for the Westy kitchen/fridge but poorly
if asked to charge a battery.

The stock aux battery wiring works fine for charging the battery but few
vans have this stock wire.

The stock fridge wiring works poorly when converted to charging a
battery without adding a separate charging wire from the main battery.

Mark