We live in Kalispell, not too far from Spokane, and bought an ADCO cover last year. This will be our second season with it. It was on our trailer for 5 months through the snowy winter and worked well.
I was very impressed with the material on the cover and the way it secures with flexibility to adjust for a tight fit. Prior to using it the first time, we cut four 3 inch pieces off a pool noodle to attach to our rain gutters. Unfortunately, one came off over the winter and punctured a small hole in one corner of the cover, so we are going to secure the noodle pieces using saran wrap or duct tape to the trailer and repair the hole in the cover.
Overall, the ADCO cover worked well, though I don't expect to get more than 5 seasons out of it from reviews I have read.