We use a PermaPro cover from Classic Accessories. Festus is correct that you are lucky to get 2+ years from one. The good news on our case is that Classic Accessories has been AWESOME in honoring their 4 yr warrantee. In fact, we're on our third one of their covers but we've only paid for one of them. The first one failed after about 2 years (we cover year-round) and they replaced it. The replacement cover also began to degrade within its warrantee period so they replaced it too.
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Since our TT is small, the AC unit raises the center of the cover such that it has a pretty good pitch in all directions to facilitate rain run-off, that is until winter comes and the snow pushes it down to the roof.
I know that there are strong opinions on each side of the cover issue. I decided to cover because I hated what the sun did to my last TT. I've devised a method that allows me to cover and uncover by myself in about 20 minutes without having to get on the roof, so it's worth the effort IMHO.