Check out CalMark covers. Mine has held up very well over the last 5 northern Illinois winters. They use very high end material and have a 6 year warranty, nothing at CW even comes close to the quality.
The website has lost of good information about their covers and you can even request a frabic sample, you will see their fabric is very different than what the other cover manufacturers are offering.
My parents have also been using CalMark covers on their trailers the last 5 winters in central Minnesota with very good luck. My dad use to only get 2 winters out of the other covers he was buying from CW and Amazon.