I agree with the other 2 posts....I also live further north than you and we get snow in Nov and it stays with us till spring melt. I have never covered either and seen many who have who have suffered more damage by covering. Wind will be your worst enemy, and along with wind comes dust particles than can get lodged under your cover and the constant wind will cause some movement. Ours needs a good wash in the spring, gets a good waxing before onset of winter, and in spring is just fine.
Some people swear by their covers but I think factors to consider are wind where you live and humidity levels. Some have to be concerned about sunlight but in winter in northern climates UV rays are not a factor. With regards to snow load...our trailer is stored in a very open area in winter and I check up on it a few times over the winter and the amount of snow on it is minimal. For some reason it just seems to blow off.
2011 GMC Sierra 2500HD
2014 Rockwood 8289WS, Diamond Pkg.