I have a 2004 BT Cruiser 5210 with the same setup, it seems.
Can’t get a battery filler in to maintain the levels. Even cut the handles off a filler and epoxied the holes to make it fit. Still not quite enough.
When I bought it last year the battery was bad. Got a new one and talked my son into bench pressing the new one up, while tilting it on its end. Kinda dangerous.
Anyway, I want to find a welder who could help reconfigure. Get rid of the diaganol brace blocking the entrance door. Maybe a drawer? Need a bigger hatch to accommodate the drawer. I was thinking of a slightly larger hatch, like my black/gray water access. Have to find an RV salvage place.
At the same time, need to address the wheel well right behind the battery. There’s no partition between battery and wheel well so water throws onto battery.
I hope they don’t pay people to design this stuff.