Coachman major repairs

I would not bond two 1/8" pieces together. That just leaves a chance of those pieces delaminating. Use a single 1/4" piece (or 5mm is close). i recently redid a Montana rear wall and used 5mm plywood from home depot, and contact adhesive. Used the old wall as a template and a jigsaw to trace and then cut it out. Laid the filon on the garage floor upside down and then rolled on the contact adhesive. Did the same to the plywood and then bonded them together. Use a roller if you can, if not step all over it evenly and then use some heavy objects laid on it overnight as a minimum, or a few days if you can. I used landscape bricks. Get a couple guys to help position it in place on the camper and drive your trim screws into place. On my unit there was no glue used to actually hold the wall, it is basically held in place by the 4 trim pieces at top/bottom and sides. The lamination is what gives it structural strength. It was not bonded to my aluminum studs in any way and actually came off easily once the 4 trim pieces were removed. Dip all screws in sealant before you install them, this stops any water that could wick in. You'd be surprised how many of your exterior screws are rusted already due to water. They don't do this at the factory and is a primary place for water damage to start.