I might have room for two batteries behind that original door if I re-located the shore tie cable storage. And that wouldn't be all that hard. It passes through the back of that compartment then across the bottom of the house till it gets to the passenger side of the bed frame. The Converter/Power Center is located there.
All I wanted that my setup lacked was a sliding battery tray. I have a pic somewhere of the mod I made using two Heavy Duty full extension slides from Home Depot. I had to shim the space from tray to frame down a little and found plywood scraps that'd do it. Now I can check and water the battery without having to disconnect AND pull up/out from that deep tray. Worse part of the job was getting the self-tapping screws out of the four corners of the tray. Best part was it justified buying a rigth-angle cordless drill to get the slide mounting holes through the sides of the support rack. I'll look for the pic or take another one.