I have a non-Westfalia conversion that has a aux battery in it. At one time,
it had a battery isolator in it - but this was apparently causing some sort
of drain on the main battery. Before I got it, a mechanic's note indicated this
along with the info that he had replaced it with a simple EM relay, as is done
in later model Westfalias.
I looked all over that for that thing, never could find the relay - but did
find the isolator sitting there in non-configured mode. I even found the
mechanic that did the work over 3 yrs ago - he says if he put it anywhere
it would have been in the engine compartment. It's not there as far as I
can see. So I put one in myself. I'm not sure if the aux battery is getting
as much juice as the main battery, but it seems to be charging...
So much for the leadin.

Hooking up the AUX battery is shown in the Bentley manual (look for 1986
camper electrical section.) Basically you put
an X-circuit switch in. They carry these at your FLAPS - they even have the
correct numbers to match Bosch electrical terminals on the back. You need a
30-40 amp device.
The blue wire that feeds from the alternator to the dashboard light should
be spliced, with a line leading into the EM relay, I think terminal #85,
with #86 wired to ground. Then run a nice big cable (10 AWG?) from the
charging post that feed the main battery to terminal #30. Then run a
similar size cable from terminal #87 to your AUX battery.