Iishka - I guarantee for every reply you get praising covers, you'll get one saying they are the work of the devil!
We keep our van in a storage site a few miles from home and keep it covered over the winter. Never had a problem with damp, most covers are made from breathable fabric. It is a two person job to get them on. I use two extendable poles with tennis balls stuck on the end to help lift the cover over the van (particular care required over aerials, chimneys etc. Keeps the muck and bird droppings etc off, although we still get some light black streaks but a quick spruce up is all that is needed when the cover comes off.
Look for ones with a soft inner surface - like most things caravanning you get what you pay pay for - some covers are tailor made, some are a generic size range.
Thumbs up for covers from us!