I finally got around to replacing the 12 volt
batteries with two six volt batteries. Got two Trojan T-105's when I was in Los Angeles last weekend. There were a much better price there than at home. These replace the original Airstream Interstate group 24 12 volt
batteries. I trimmed the plastic holder at the bottom of the battery box to allow for the slightly wider dimension of these batteries, otherwise it fit perfectly.
The project was not totally simple as I ended up replacing the battery cables with marine type. The other cables had some bad corrosion and they had the wrong terminals, automobile type. I'm happy with the result. Usable amp hours from up from 80 to 112 (total Amp hours of 225). We used to get about three days camping with the original set, this should give us about five. I have a solar setup so doubt I'll ever have a power problem.