It's more likely dirt and chemicals from the air than oxidized fiberglass. I get it too if I don't have a chance to wash and was occasionally. I use a wipe-on floor wax like Mop n Glow - you don't (or want) need a glossy polish, but a wax will help shed the dirt for awhile.
It's ok to paint the roof - any good quality paint will do - but it may not alleviate the streaking.
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL