Most caravan covers last about 2 years if they cover the van 95% of the time.
I have tried better more expensive ones like ADCO (double the price!) and it last about 6 months longer and then I claimed warranty and had it replaced.
One tip I would give it to make sure there aren't any sharp points on the van that will make the cover rip in the wind. If there are cover them with foam or a towel underneath.
Daddario EJ27N Classical Guitar String Set Nylon Normal