Chas, I happen to have had quite a large empty area near the rear step of my 225, good thing to know is there does not seem to be any structural Alum studs below the floor level around the coach. So it's just a simple cut with a jig saw to the size of the hatch trim.
How has your stock battery tray stood up? As mentioned before I had to replace my original one with a Stainless ( acid will not eat Stainless ) I would remove your battery and take a real good look at the welds holding the tray on. My new tray also has a stainless hold down clamp!!! If you recall a few years back I lost 2 brand new Interstate Deep cycle Batteries on the Jersey Pike after the plastic clip on the nylon hold down strap broke, 1st bump I hit lost one battery with the 2nd battery hanging from the neg cable. It dragged for so long that a 1/4 of of the battery case was worn away. What a nightmare!! Must have driven over 50 miles not knowing they fell out. Thank God for those 24 hour Walmarts.