My Stealth Camper is built with FRP and it works fine. I didn't build the FRP box though. It was already built and on the truck when I bought it. I think it is made by Supreme Corp.
It seems to me that the FRP is a pretty nice material. You can work with it using standard wood working tools. In my experience it has a very tough and long lasting finish that is impervious to most solvents (handy for cleaning grafiti or removing vinyl stickers). The fiberglass does make a lot of itchy dust when you cut it though.
I have seen complaints about it delaminating if moisture gets inside. It seems to me the solution to that is to not let moisture get inside. That's just a matter of proper caulking and well considered designs with no exposed edges, which implies the use of the proper aluminum extrusions at corners. From what I can see the through hole fasteners are also somewhat specialized to insure a proper seal.
I have seen complaints about FRP being hard to invisibly repair. It's true, if someone punches a hole in it then you can either patch just the hole, which will leave a visible seam, or you can replace an entire panel. I patched such a hole, and you can see how it turned out in the pictures in the photo gallery under keyword "stealth".
You mentioned using FRP on the roof. I am not sure if that is a good idea. The water incursion and delamination issue might be harder to mitigate on a roof, especially since there are usually antenna, AC units, and other holes through the roof that could cause problems. My truck uses a single piece sheet aluminum roof which seems to be a life time roof given proper maintenance of the caulking every few years.
Your truck model looks really nice :-)