Gaco Western (Firestone Building Products),
here-->...that sounds like a very interesting and robust roof build-up! I'll probably use Gaco to 2nd skin my TPO roof down the road (perhaps in 7 years....BEFORE a roof leak happens). Have you considered just using 3 sheets (do the cut-outs) of marine ply (12-foot sheets for sidewalls all the way out to tip of cabover, and scarf lower & scarf a couple of smaller ply sheets to form the rear wall) and doing the same build-up as the roof (as a smooth camper "skin") ?
...also, check the under-wings and camper tub and tub under-floor for rot, too. I had to re-build mine a few years ago...also, it appears (in your photo) that the cabover underbelly could be delaminating (??)
