Rob and Bob,
Thanks guys. As I typically do, I decided, just prior to pulling out the oak board, to make a run to the lumber store.
I realized that ply really is the only choice simply because of the dynamic strength issue. I assume that there would be stresses on the rear-end that I could not interpret. As hard and weather resistant as Rift Oak is, it still has a grain and could split. Ply would not.
I paused this morning, remembering spending 3 long days sanding all the bathroom fixtures to one color, starting with 200 grit ending with 1000, then going through a polishing step. All that work, tear-out and soon the re-assembly of the bath would be undermined by a cracked board on some boon-docking trip down an uneven road, sure as shootin.
One benefit of getting older is the gift of patience. What matters most is to do it right.
I finished it today, all new, with a .125 stainless hinge plate this time for the rear storage door. Much more rigid and solid than the T2 .040 soft aluminum strip it did have which was powder everywhere it met the U channel.
The patches looked great and fit nicely, I used foaming polyurethane glue which expanded nicely into the gaps well above floor level, which I was able to sand off smooth without having to come back and fill those gaps between the patch and the main floor. I also painted the marine ply on the edges and where it goes under the channel on both sides with a black oil based enamel.
I replaced the old rotten right angle support piece in the back with marine T6 1/8 inch aluminum instead of the mild steel that it did have.
Everything (all the new parts got a mild muriatic acid wash (8:1) and POR-15 prior to reassembly. All riveted and sealed. All seams at the back have a thin bead of Vulcum and the interior wall got the same rigid white rubber based sealant, similar to what the factory used.
Turned out great.
I am so grateful to all the threads I have gleaned information from and for the kind replies from everyone.
I have a series of pics I have been taking but have yet to organize and load. Remembering to take them in the heat of battle though is the toughie.
Thanks guys, we never have enough time (or money) to do it right the first time, right ? but always have both the second time around. If I learn anything during this restoration, it will be patience. As one member advised me early, it is a series of individual, small, but achievable projects. One at a time.