RE: Whats the Advantage of the aux battery....
Although the aux battery will not handle the fridge for very long it
allows the use of 12v on the fridge while traveling, then immediately
switch to propane or shore power if available.

Many people add addition 12v outlets and wire to the aux battery to
run TV sets, stereo, laptops, etc.  In addition to the re wiring of
the interior lights, etc. already mentioned.

To find out what may have been added to the aux battery take a look at
the wiring on the positive terminal.