We ordered a Classic Accessories OverDrive PermaPRO Deluxe Travel Trailer Cover, Fits 20'-22' RVs. I called Classic Accessories and they verified the size for me as I was concerned (pre-purchase).
Anyone who decides to use a cover should also order the RV Cover Rescue kit by MH57 Products. It's ~$26, includes two pairs of round plastic balls that cover the rain gutter spouts along with straps to main tension when installing/removing the cover. I added an extension strap for the rear pair as the default strap was not long enough to secure it to anything on the roof. The rear strap wraps around the MaxxAir II Smoked Vent Cover that I installed over the bathroom vent allowing us to keep the vent open if it is raining.
Without the RV Cover Rescue kit, your expensive trailer cover will potentially have tears on the first install/removal (from experience). This is totally different than a car cover as cars/trucks do not typically have sharp edges or extreme obstacles such as the rain gutter spouts. The sharp edges of the aluminum trim (bottom four corners of the trailer) will also put holes in the trailer cover. We plan to add additional material to the problem areas on our cover to address this.
So far, having the trailer cover has kept it cleaner than not between our trips. Not a big deal for most trailers, but the V21/V22's are show trailers and I want mine to look good and clean without having to spend two hours washing/drying it before a trip. We live SoCal, so moisture is not an issue as it is fairly dry here. If I lived in a wet area or even dry windy area, I would might rethink the cover idea. High winds slapping the cover against the trailer will scratch the siding and will be overall worse than no cover.