Hi There,
We used to have a universal cover for our first caravan. As others have said, they are wonderful at keeping environmental fallout off of the van, and indeed keep it clean, but as mentioned, a little bit of grit or a picked up thorn can easily damage the van particularly the windows. I didnt have any problems with damp or air not circulating etc, BUT we found it to be a monumental pain to put on. (The first awning Mrs K and I put up seemed like a doddle in comparison!) We found two approaches worked the best:
1, make up some form of pole (i used two old decorating roller extension poles) with old tennis balls on the end to lift the cover front to back. It becomes quite heavy about half way down the van though! If you have straps that go under the van from side to side, the poles also help for this, but be aware that it can be easy when tensioning these to damage wheel arches/skirts etc on the van.
2, roll up the cover, and using steps place it on the roof of the caravan and unroll.
Our cover used to catch on absolutely everything, to the point that when the booster on the Status 315 packed up, I removed the aerial all together to save getting up above and removing the pinnacle. (two guesses what i Used to forget EVERY time we went away too!)
We were quite lucky inasmuch as we could store the cover at out storage site (family owned) but if we had to take it home, without the storage bag it would consume vast amounts of space in the car!
Our cover met an untimely demise when the electric fencing fell down, and a horse decided it was the opportune spot to deal with the itch on its rear end - suffice to say, it snagged the awning rail that you put the draft skirt into, and ripped.
We bought a newer van, and essentially keep it at home now and isnt so susceptible to sap and suchlike.We also go away quite often usually atleast once a month.
Overall, if we werent able to go away as often, I would be tempted by a custom fitting cover, atleast to save the paintwork, but not for how we use the van currently, as I couldnt muster the effort to replace the cover every time we came back! Appreciate this is an 'on the fence' post, but might have some helpful info for you.
Another member mentioned UV protection - for what its worth, I use some covers for my tyres for when the van is parked to try and slow the UV damage to the tyres - might be worth looking at too?
Cheers
Rob