RE: What's on the aux battery?
Your signature says you are driving a Weekender.  If the aux battery is
wired according to VW/Bently the only thing that will be on the aux battery
is maybe a 12v outlet.  The rest of the interior lights, radio, etc. will
still be wired to the starting battery. If, when the aux battery was
installed, a wiring change was made, then the accessories could be on the
aux battery.  The easiest way to check is after you have a good battery
installed for the aux, with the engine off and the key out, turn on the
radio and the interior lights.  Then disconnect the ground strap on the aux
battery.  If the lights go out and the radio dies then they are running on
the aux battery.  If not then they are still wired to the starting battery.