Multiple answers:
For nearly all the US a cover, which is in contact with the RV can do more damage than good. If any dirt or grit gets under the cover, which moves slightly as the wind blows.. You have SANDPAPER.. If you wish a better understanding take a nice piece of polished metal, or well paited metial and apply a sander for a few minutes.. (Actually do not do this, just imagine it).
On the salty coasts... Well,, Decide
Second: Indoor storage.. A cover (building) that requires no application (just back or pull in) and which does NOT contact the coach, no sandpaper, no wind, no sun, no damage (Save rodent) costly
Finally what I do. Neither, Of course I full time.. RIG still looks (Save for some damage) new.. Finish is still shiney, 11 years and 22 wax jobs later.