I purchased a Classic Accessories light weight cover the first year I had my 5th Wheel. They are made right near Seattle so I thought they would know about a cover that is water proof. I found out after the first rain that the cover was not waterproof. Everywhere the cover touched the RV it leaked through. Much like a tent. I even had standing water on the roof. Fortunately I was able to take it back. It would be good for sun block but not for wet weather.
I ended up purchasing an Adco cover. I have had it a few seasons and it is working fine. Water or dampness might get underneath the cover but no pooling water and no issues with mildew or anything that might damage my trailer. I do put some buckets upside down on the roof where the cover sits lower than the AC and then water rolls off the cover. We have had snow on it, up to 18" and no issues with the cover.
Because my trailer is older the roof is not in the best shape but the cover keeps any water from getting onto the trailer and gives me piece of mind with the amount of rain we get. I don't think it is waterproof since it is made of materials that breathe, but it is better than no cover. I also have not seen any damage from the cover, the straps and such do not damage the side. You do have to keep it tight so wind does not blow the cover around. On the plus side, I wash and wax my trailer before putting the cover on and it is clean and ready to go in the spring.
I would definitely recommend a cover even though it is a pain to take it on and off. I would use a garage, but there is a huge price difference between a cover and a permanent building...