"Some sort of plastic paint? Sand down and spray with a paint designed for RV plastic tops?"
Yes. Rough sanding usually. Followed with a microfiber rub, than paint designed for fiberglass.
Fiberglass IS plastic.
The roof seam caulking? I hate caulking myself, never have been patient enough to do a good job with it. For that reason I love Eternabond Roofing Tape. Stuff sticks like crazy, is darn well impervious to everything thrown at it on a vehicle's roof. Available at Amazon in a couple colors. There's several widths available. Home Depot carries it too, usually in the 2" width.