Both of the Westfalias I've owned came with an aux battery installed under
the drivers seat.  Never paid much attention in the first one, my '85.  But
in my '90 I found out the hard way that the aux was essentially hard wired
to the 'fridge, would run the aux dry in about two hours.  I now have a much
more complicated setup in my '90 that I understnad, not at all stock.
But I've just been thinking about the from-the-factory Westy wiring (no aux
battery in place) where the 'fridge can only be run on 12v with the ignition
on, engine running.  And then trying to remember what the configuration is
IF and aux battery is installed, with a relay added, according to the
Bentley wiring diagrams.  My understanding is that the 'fridge is, with that
install, still only going to work with the engine running.  And I know that
all the accessories will still be on the starting battery.  So, what is
gained by installing an aux battery (according to Bentley)?  What am I
missing, I obviously don't understand (or misunderstand) something.