It sounds like Eric is considering a project similar to what Westronics (Bill, a former Moderator on this Forum) did on a Jayco Granite Ridge 24SS. Bill found unused space next to his battery tray and installed a wider, slide-out tray from Kwikee. Bill is one of those meticulous guys, and he posted a thread about the project. I've searched a several times. Should be Author Westronics, Forum Class C, Archive, then Battery, or Tray... but I could only find where Bill mentioned the project. Not where he documented it. When he was done, it looked like it came from the factory. No, actually BETTER than what most "factories" do. It involved enlarging the cutout in the side wall of the coach and fitting a larger compartment door and frame.
I worked in there on our Jayco Escapade but all I did was add slides to the existing tray for a single Group 27 deep cycle battery. I bookmarked that thread and can link a picture of the area. You can see wasted space behind the wheel well, and how the size of the power cord compartment could be reduced. The smaller compartment would add a little battery room and still work. Then the rest of the space could be picked up on the wheel well side of the battery.
Wheel well is to the LEFT of the battery. Notice the Battery Compartment is not a compartment. It's only a bracket hanging from the coach floor. You can see the ground under the battery side of that "compartment." Door covers Battery and Cord Storage.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB