Rather than a tarp, which is abrasive and not breatheable, which you need both...check into an Adco. Camping World has a huge fall sale and they can tell you which one is right for your area. In our neck of the woods, we cover from some time in Nov until about Feb, or whenever we're getting ready for spring training in FL. Covering keeps it clean, protects the decals, protects the roof.....however, I wouldn't want to do it every time we camped. It's about an hour or less to do the whole thing, start to finish. But it's bunky and downright inconvenient for regular use. Ours is for winter protection. We store ours in a covered area 24/7, but still do the Adco for winter.
As far as leaving vents open, we don't. We store at home, so we have electric and just leave the A/C about 80+. If you don't have that option, you may have to open a vent, but be sure you have the rain cover so it doesn't end up raining inside. Also, you might check out Damp Rid....works well.