We bought a Classic "Deluxe Poly Pro II" cover for our 25' TT back in 2008. It's the gray one with the white top it has CLASSIC printed on the front of it. It's a cheaper model, it started to break down significantly after the 2nd year. I patched it up for the 3rd year then bought an Anco Designer Series Tyvek model. This second cover is much better. After the first winter there were absolutely no visible signs of wear. It is thicker (3 ply vs 2 ply) and feels softer. It is sewn together much better. All the seams are reinforced. Also, in my opinion, the sun is just as damaging as the rain/snow. The sun over time detiorates everything. So I cover our trailer all year long.
Really shop around for a cover if you buy one, prices can vary greatly. And tie it down tightly. I had no problems after covering out last trailer for the better part of the 4+ years we owned it. We just took delivery on a 2011 North Country 22RBQ and I have the cover on it right now.