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 in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC