There are a ton of threads here about covers, and you will find those that love them and those that don't. Since we weren't sure a cover was going to be worth the trouble, we bought the cheapest one we could find. It as a Tyvek-like non-woven material (probably not real Tyvek), and while we loved the way it kept the trailer perfectly clean and critter free, it completely fell apart after a couple of years. We replaced that with a premium cover from CalMark made from Sunbrella woven fabric. It was so great, we purchased another CalMark for our new trailer.
I don't know if you really need a CalMark if you can find one of the "standard" size covers that fits well - most the cover problems seem to stem from a bad fit. Several of the manufactures offer Sunbrella fabric and I highly recommend it.
Bill & Kate - Stone Harbor, NJ
w/ Bailey (standard poodle) and Zeke (partipoodle)- both rescues
2018 Ford F-250 Super Duty Crew Cab w/ 6.2L gasser
2014 Forest River Wildcat 272RLX fifth wheel