I am tying to figure out the feasibility of that as we speak. I just crawled out from measuring battery box area. I would need to tear out the factory slide out box, fabricate a new one that may or may not have a slide out tray. My desire is to have 2, 6 volt golf cart batteries with appx 205 amp hours. There is room for them if I can get a tray fabricated. I am neither an engineer nor a fabricator. I worry a little about the weight of the batteries and whether a new battery tray in place of the old one would pull the mounting bolts through the floor.
Another option is under the rear bed if you use sealed batteries. That adds more complication with battery switches, long cable runs and possibly uneven charging issues.
So I am, thinking about it. My first step will probably be a new group 31 battery for short term. Even that will require a little mod to the battery tray. We lose some space because of the rivets on the inside of the tray. Our "26" model coaches don't leave us a ton of options.
I happened to notice on a 2019 Redhawk 22J, they have moved the battery to under the entrance step, like most others. Looked like room for 2 also. Maybe Jayco is starting to listen.