If you can afford the tongue weight, you can normally fit at least 2 size 24 12V or 2 golf cart 6V batteries lengthwise across the a-frame of an A-frame or PUP. My dual battery box has an inside length of 22", and an inside width of 7". It just fits inside the a-frame behind the dual propane tanks. There are a pair of metal rails that support the battery box that run across the a-frame. I'm not sure whether the rails were standard from the factory or added by the dealer, as I bought dual batteries as part of the deal on my Rockwood A122.
I had learned dry camping with our 2000 Coleman Westlake PUP that the single size 27 battery was my first resource (propane, water, food, electric) to run out. When we bought the A-frame, we wanted to be able to dry camp for 4 nights in the Colorado Rockies for 3 seasons (winter, too if no snow at campground). Heater set to 55 at night (27+ degrees), minimal other use of battery. With the dual GC batteries, battery runs out about when water does. Depending on heater use, I can go a week or more on the batteries.
As I said, tongue weight is often an issue for PUPs and A-frames. You are adding another 60-80lbs of tongue weight for the 2nd battery. I use an Equalizer E2 600/6000 to tow with my Hyundai Entourage minivan to resolve any tongue weight, rear end sag, and sway issues.
I have learned the hard way that two 6V batteries have more capacity, and last longer than comparably-priced 12V batteries. The 6V batteries are slightly taller and weigh more than most 12V. I bought my golf cart batteries at Costco for $150 for the pair. The 6V are rated at 232 AH, the dual 12V marine/deep cycle were rated at 170AH (85AH each). Of course, only half of that is usable without destroying the batteries.
I would also add a shut-off switch for whatever dual battery configuration you use. It makes life a lot easier when storing between trips to just shut off the batteries. My shut-off switch is a marine dual battery switch mounted to the end of the battery box.
If I can figure out to post pictures on this site, I will upload photos of my installation.
2014 Rockwood A122
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time