Just in case anyone else has wanted to do this:
I bought a 12v cooler, principally to take home the contents of the fridge when we stored the Beaver in Lodi, CA for a few months over the winter while heading home for Christmas. Then when we returned to the coach, I found this to be a great beer fridge, so needed to be able to plug it in while stuck in the storage bay. I at first thought I would need to fish a wire from the battery bay to the storage bay, but getting past the rear axle looked difficult, if not impossible. I then crawled into the storage bay and hunted around, and found a wire loom looped over the black water tank (just behind the storage area). The printing on the wires was "DC to rear of coach", so I cut into it and wired in a 12v outlet that I installed beside the 120v outlet above the water system controls, all at the back side of the storage bay, just inside the big door. Saved drilling holes, fishing wires, etc. Only downside is that I don't know exactly where this comes from, so I don't know what else is on the circuit. Haven't blown any fuses yet, so I don't think it is carrying any other loads.