My view is that the covers you see in a box at RV supply places which are vinyl, either black or white, may do more damage than good. However they are still marketing so maybe the damage I was told about does not happen. I was told, I think here, that the contact points will transfer heat and damage that spot. Not sure of the veracity of that one though. If you are stored for a long time then the sunside will definitely age faster.
Towards that end, we are getting covers made this week. the same material used for the windshield covers, allow air to flow, and no contact. Snaps on side and hangs to groound. Keeps sun off, the material blocks 90% or so of the UV. I think they look a little better too, the drape over covers always look rumpled.