We have a cover that came with the private purchase of the fifth wheel. I have used it two winters, and because of where the RV sets and the resulting wind that is nearly always blowing from some direction, I have also used ropes around the RV to keep flapping to a minimum. Yes, I know this is not "properly installed", but it has rubbed four spots in the roof where the white oxidation is off the rubber roof. I have patched these places with appropriate materials (I just don't remember the brands right this minute) and Dicor sealer around the rounded off edges. This does not look good, in my opinion, but didn't know what else to do. Don't want a problem to begin, if I can help it.
I also plan to replace this roof at 15 years (its an o7 Jayco) with a spray-on material such as Rhino liner, which I have had very good results with in more than one truck bed. I hate the black/grey streaks, but they are impossible to eliminate with the current roof set-up.