I bought a Classic Accessories cover from Amazon for ~$120 and it lasted three years. I just purchased a replacement poly version, which should last even longer, for $139.
We have a lot of wind and rough Wyoming mountain weather here, and it held up very well for the price, I think.
I missed covering a couple sharp edges with duct tape or it would be good for one more year. I put old sox over the ends of the TV antenna so it wouldn't wear there. That's the key with the cover, cover the sharp edges, and don't forget the fenders.
Last winter we had a record over 200 inches of snow and high winds and it was literally buried for months. The top held up very well, the sharp edges were on the tracks for awnings that I missed, and being stretched out like it was from Nov thru March it cut through when I pulled the cover off this spring.
Always better to go indoors with it, but I'm too cheap to rent a place, and it has always come out nice and clean with the cover.
Also, I wash it with RV wash I buy at WalMart, and use a soft brush on a long handle to scrub it clean, before I put the cover on to make sure there is no dirt or dust under there to scratch the finish. RV wash really puts a shine on there, with very little effort.