I would definitely go with Tyvek. I would stay away from any covers made by ADCO brand -- the one I bought was junk. While we were having our carport rebuilt, I bought an ADCO cover for our travel trailer that was supposed to be "sun-proof," with a supposedly Tyvek top (but I don't think it really was). After less than 1 year in the intense Arizona heat, the cover literally disintegrated. Now, we store our travel trailer under the new carport, which blocks out 99% of the UV.
Meanwhile, I purchased two "BUDGET Premiere" covers for our two cars made from Tyvek and they have held up really well. The Tyvek material is strong, lightweight, water resistant, breathable and most important (here anyway) sun-proof. It blocks 99% of the UV rays. So, if I were shopping for another travel trailer cover, I would definitely go only with Tyvek.