I had my first cover for 5 years. Just bought the new one this year. They are great at keeping the RV clean and depending on the cover, dry. My first one was a standard cover from Addco my new one is a SFS Aquashed. The Aquashed is more suited to wet climates. When you do use it be sure to pad all corners and anything that protrudes. I use bubble wrap on all corners/edges. If you follow that the cover will last a long time and keep the trailer looking like new.
BTW, the cover cost me $250 and i get 5 years from it. Indoor storage costs me $400+ a season. So even though it's not truly out of the elements, i'm ahead when using the cover vs. indoor storage.