GH, I have made aircraft accessories for years and your worries are really on the outside. It is UV (ultraviolet light rays) that cause the most damage. (Did you ever see those old guys, the 'Suntanners', in FL in the 60's? ...'same results.)
Not only does the ceiling not collect dirt like flooring, I cannot imagine a situation where it gets much VU exposure... 'there may be one? I wished Winnebago had used a smooth surface, but a once-a-year vacuuming might make me feel better about dust.
Scotchguard, 303 Fabric protection probably will not do much good or bad. The carrier evaporates, so there's no real weight added. I'd suggest you get UV blocking shades (in or out) and keep them drawn - especially when you are not using the coach. As soon as our weather improves - I'll be making some for mine.