Teardrops n Tiny Travel Trailers

I used this resin to adhere to my Fiberglas when I built a surfboard last summer and I have A LOT left. waterproof, UV stabilized, and nonyellowing could it be okay for the exterior walls of a woodie teardrop?

I don't see why not. I used surfboard laminating epoxy on my build with fibreglass cloth over XPS foam. For a woodie you could then use 2k clear coat that is compatible with sanded epoxy to make sure it is protected against UV. I am not confident that any epoxy can stand against UV long term, but 2k clear coat will as this is the coating used on cars. I have no experience with spar polyurethane over epoxy, but Friz here would have done that, look at his "Sin Bin" build.