We got a cover with ours when we bought it. I took one look at the weight and hassle and decided not to use it. Our rig's manufacturer specifically said "DON"T walk on the roof". I was a rubber membrane type. Why they installed a ladder on the back to let you do just that was a puzzle!
In western Washington, fading due to sunshine isn't a problem. Mold and mildew are, and I figured there'd be more of that under a cover than if I let our semi-perpetual rain keep washing it off.
Bottom line - it depends mainly on the climate where your RV is stored. and whether it's under a roof or not.