I think a proper RV cover is a good idea and used one years ago on a smaller Trailer. It helps keep it cleaner and protected to a point. I used a blue tarp one year just on the roof and a bit over the sides. It did not work out well at all. Scuffs the finish and doesn't breath at all. Did more harm then good perhaps.
I want to cover my 5th wheel but at 36 feet I don't think I can do it alone. I don't really have anyone nearby to help with hauling it up on to the roof. The covers can be heavy and unmanageable for one person who is at retirement age.