Extra wide trim molding?

Making fiberglass components in a mold isn't super difficult, though it is time-consuming and a little messy and can get a bit expensive. There are plenty of videos etc. available. The usual approach for something like this would be to form an outside mold with whatever sort of materials you can readily work with (wood, silicone, or whatever), painting in some gelcoat, and then adding layers of bits of fiberglass cloth or matting with your resin of choice such as expoxy or polyester. (The parts can also be built from the inside out, but from my limited understanding it's harder to get a nice finish that way.)

Fiberglass or similar techniques are used for many things other than RVs; there are some good tutorials to be seen aimed at e.g. cosplay aficionados or boatbuilders.

House trim parts are not a terrible option, as you already suggested. Sometimes it helps me just to wander through a store and see what's available. Perhaps a piece of vinyl or metal guttering could be trimmed lengthwise somehow to work? Or maybe an aluminum angle extrusion?