Hello Dan and Deb,
I've done a lot of fiberglass work as a surfer, so I'm not intimidated by the custom shower and pan. If you can find a shower pan that works, though, go for it. It may work in a side shower, but probably not a rear-corner. You'll probably want to use Fiberglass-Reinforced-Plastic (FRP) panels for the walls, but you'll have to ensure a good seal at the seams. Mine is seamed, but only in the horizontal direction, where I can overlap the panels by an inch or so.
The solar cells are Unisolar PV Laminates (http://www.kyocerasolar.com/pdf/spec...s_laminate.pdf
), designed for standing seam metal roofs. They simply adhere directly onto the metal, they're flexible, and they come in different lengths to suit your roof. My plans indicate 12'-6" long modules, which are 87 watts each.
Airstream shells are "monocoque", which means the frame relies on the sandwiching of the two aluminum skins. The skins are diaphrams. My endcaps are fiberglass, not aluminum. As far as the option of using different types of aluminum, and their characteristics, I have no knowledge to pass on, sorry. You may try a "general exterior" forum on this site.