I used to cover and it never did any harm, but I never really saw much benefit either. On the upside it's covered and somewhat protected from the elements. That said, RV's are designed in way to withstand the elements, especially modern finishes.
On the down side they are no fun to get on or off, they can rub in the wind, they rip pretty easily and they are even less fun to get off in bad weather and impossible if you need to uncover it with snow or ice. Learned the hard way when we decided to take a spontaneous trip south to escape cold weather and it was frozen to the roof.
I don't cover any more and frankly I can't tell a difference. I do wash and wax regularly.