John, You ask a very good question.
If you look at older PCs for sale on the internet, the ones not painted having raw plastic parts like the side mirrors, stove vent, things like that tend to "fade yellow" fiercely from the sun. Then the rig does not show as well.
Also consider that unpainted fiberglass becomes hard to look good over time. No matter how determined you are, the surface will get chalky and then harder and harder to keep looking as white as before, staining quickly from rain streaks too.
If plain old white is your color, I would consider having the entire rig painted white. The painted surface will be much easier to maintain, the white color would reflect the heat of the sun best, and it resists fading most. The question to ask is....how are the dark colored full body paint jobs holding up to the sun? Our 9 year old rig still looks great, but is stored indoors which is not a fair evaluation.