Damatheny,
Sorry I can't answer your question but one thing to note and you may know this, on my first RV, I had one of those universal covers. The cover rubbed on the rubber roof edges as the wind blew during the winter. This caused the rubber roof to wear in these areas. Besides cosmetic blemishes I'm sure that if I would have kept it this would have been the first place the roof failed.
Just something to keep in mind if you decide to go the cover route and get one that sits on the roof, be sure to secure it well or protect areas where it may rub.