We have always covered in the winter, with no problems. We use an ADCO All Climate + wind. We keep the trailer clean and the cover tight with additional bungies and alligator clips.
We are looking to find a better cover for the rest of the year. After 2 hail damaged roofs in 4 years, we are looking for something more substantial. We have typically only covered the camper in the winter with the ADCO cover. The only other real option I find seems to be down under. . .
http://www.stormcovers.com.au/