[rickst29]

Completely re-written, eliminating the VHB tape option. His large panel can generate considerable "lift" force during travel.

AFAIK, although it may have changed after my model year - the "structure" of the shells is only the foam and aluminum sandwich, with foil-covered plywood along the edges. Probably use SS screws all the way through the roof for each Z bracket. On the inside, with fairly 1" washers to spread the force of the screw and nut onto a wider area of aluminum sheet ceiling, and then small nuts (nylon sleeved "self-locking' type) below the washer. After drilling, but before mounting the Z-brackets and inserting the screws, I would fill each screw hole with 3M-5200 boat sealant and also paint the underside of the Z-bracket with the same stuff. When tightened, that will provide excellent waterproofing - wipe away the excess which gets pushed out (forming a caulk-like seal along the edges of the Z-brackets, and also at the screw entry into the roof).

Inside, I'd wipe off everything which gets squeezed around the washer. Then I'd hacksaw any remaining screw length which extends past the nut. I'd use nylon sleeved 'self-locking' nuts in this application - If anything ever feels loose (and it probably will need some tightening after the roof sandwich settles a bit) the screws can be tightened from the outside top, and the nuts shouldn't move.