There certainly are options for battery location in a westy!  The PO of my westy built a battery box (fiberglas covered plywood, with a vent to the outside) under the rear seat.  Inside are two golf cart batteries, for a total of around 170 amp hours.  When I replaced them, they were about $60 each, at a local farm supply place.

He also replaced the fridge with a Norcold electric model (compressor based, like a home fridge) that run from 110V or 12V.  It has much more interior room than the original, gets cold enough to make ice, cools down quickly, and runs for 3 days from the battery.

Bill