Here are a few links:
http://www.roadtrekboutique.com/product ... tegoryID=4
For the Roadtrek I had I got the cover sold by Roadtrek Boutique. It was actually made by Hugger Covers but they could only refer me to Roadtrek Boutique because of their contract at that time. I liked it but really only used it when my Roadtrek was not going to be used for at least a few weeks.
Calmark covers get recommended often by people who have bought them. They have a cover that might be better suited for lots of sun.
I don't know much about the price of any of them.
Note: Taking off and storing a wet cover is not fun. These things are large. I used to pay attention to rain forecasts and make sure to get the cover off when it was dry even if it was a few days before a planned trip.