Of course Ron is are right, the AC system is independent of the 
12 volt stuff. There are some neat ways to connect them up in a Vanagon
Westie. 

The first is a *small* battery charger, which might fit nicely in there,
somewhere, under the drivers seat, and charge that big, mean auxiliary
battery. That will power the rear light, the fridge on 12 volts, and the
stereo or anything else you have wired into the auxiliary battery
circuit...including the electrics for the Propex furnace, of course.
(Of course, your fridge should work on 110 also, so not much gained on
that item.)

The ultimate solution, though, is a Hart Inverter, hard wired
from the external 110 connection into the bus, including both main and
aux battery, as well as your internal 110 outlet.

When you are off the grid, it will (very quietly) convert 12 volt from
the aux battery into 110, running out of the stock outlet. 

Plug in the van, and it will charge both battery banks independently,
and pass 110 directly to the 110 outlet. 

I have friends living off the power grid who swear by these units, and
this brand. Expensive though....more than a Propex furnace, anyway.

The last alternative is to buy an inverter and battery charger and wire
them all into your Westie, for the same result as the Hart. Much
cheaper, I am sure.

Tom F.