I briefly went back , Im sure theres a picture though I didnt find, of the area at back.
But is there something that precludes adding a block behind the section of wall between door and tank access? Preferably hardwood, or maybe a piece of steel angle, but using bolts, pair thru floor & block and thru (plywood) wall & block. T or elevator bolts.
While perimeter sides or even along the front of camper using pocket screws strongest of the options and the strapping assisting, Im seeing from the back wall, the door-and from inside the bath door, simply a lot of concentrated/constant forces applied at that span. Its also only area, albeit short, unsupported when in truck.
Pocketing that small section of back wall simply subject to movement, a lot of force relying on pocket screws in ply to my eye failure, at least I wouldn't. Just an opinion.
Last comment is about down the road, using pocket screws thru wall into floor, means removing the siding to access. Printing out your posts here for the 'next' person would give them insight to how its constructed. I personally wouldn't make allowance 'ease of disassembly' Unless I though I might. Certainly not at the expense maximizing strength for a very remote possibility this camper would see a second rebuild. (or how much of that rotted wood that you removed would you have noted or mattered that it was also 'glued'.)
