Here in Alaska we get some pretty brutal winters with lots of snow. We bought a Calmark cover for ours. I didn't like the option of having snow sitting on it for months on end. I have used it 2 winters so far with zero issues. They are not cheap, but I feel you get what you pay for. Make sure you cover any sharp edges with inner tubes or something to keep them from cutting the cover. Their customer service is great and can help you get the measurements right to get a semi-custom fit.
http://www.calmarkcovers.com/5th-wheel-covers/