We have a caravan cover, and leave it on all winter - it's now into the second (very cold and wet) winter, and we've just uncovered it again at the weekend, to find it as pristine and clean (and unmarked) as when we put it away. Our cover was a cheapy version from ebay, which is waterproof, but breathable, and has a sort of softish cotton lining. We're really pleased with it, and it definitely doesn't scratch or mark the van in any way. Don't let this post put anyone covering their caravan - it's the most fantastic feeling when you take the cover off and see the van shining and clean, with no rain streaks, black marks, algae staining, or dust anywhere at all.
We did the usual checks of the van - moved it a little to take the weight on a different area of the tyes, hand-brake on and off to make sure it was still greased and functioning, checked the steadies and the wheel clamp, did a walk around outside to check for anything untoward, then a look at all the hidden corners inside to make sure nothing was hiding away (damp, rodents, etc)., moved the cushions around a bit inside the van, and turned some over, checked the battery and the gas supply, put some water through the pump and drained everything again, and then covered it back up again until next time!