Your RV is made and designed to be outdoors to the elements. A cover can promote moisture retention, some covers are designed to breathe, but will cost more. Another danger to covering is in windy conditions, you can get some movement of the cover and cause some rubbing against the finish, thereby doing more harm than just leaving it exposed. Or if dirt or sand gets underneath the cover and is rubbing between the cover and the finish, then that will do a lot of damage.
The key is to protect the finish by keeping it clean, and waxing will go a long ways to protecting the finish. Many of us here will wax at least a couple times a year.
Do you cover your car all the time?
2011 GMC Sierra 2500HD
2014 Rockwood 8289WS, Diamond Pkg.