Peter, If I'm understanding you correctly, you are talking about the shelf plywood, meaning the flat plywood under the wide portion that would be above the side of the pickup in normal use. On the driver's side, that was rotten on the outside the full length of the camper. The worst place was where the water inlets were, both to fill the tank and the city water, so something must have been leaking there. I cut out the whole piece of plywood and am replacing it. Much of the inner skin plywood is gone, all that is left is one layer of plywood and the vinyl inner coating. To replace that would have been a major problem so I am leaving that in place, I will bond it to the ridgid foam insulation, but it won't be contributing any structual strength (it was only 1/8" to begin with). The 5mm Lauan plywood will go over that, also attached with Liquid Nails adhesive, as well as stapled to the wood framework. I am planning on using Thompson's wood sealer on the plywood. I will put the original aluminum siding back on over this. Any advice on the best type of sealer to use for the seams and joints on the aluminum siding? I have heard there is a roll of putty type sealer, or the caulk gun type.
Yes, the garage will be structually attached to the bed overhang, so it won't be supporting itself anymore. Also I will be building storage areas under the side shelves, they will be resting on, and attached to the plywood decking of the new flatbed. Still not sure exactly how I will do the doors, I have a quote from the flatbed builder, but it's quite expensive (about $750), so will be doing something cheaper with wood, not aluminum.