In the desert SW where temps hit 110-115 regularly, an RV cover will pay for itself in reduced depreciation in less than 2 years IMO. UV damage here is very noticeable even after 12-18 months.
Prep work is key when using an RV cover.
Covering sharp corners and rain gutter spouts are a given to protect the cover from the RV. However, it's also necessary to protect the RV from the cover, too. Under windy conditions, the cover is going to move and rub against the RV hundreds of times a day. Most the weight of the cover rests on top of the a/c units and on the sides/edge of the rubber roof. We've found 3M painter's tape provides excellent protection on painted surfaces and roof areas where the cover rests (or rubs when the wind blows). For some reason, other brands of painters tape (i.e., Duck) won't stick to the rubber roof nearly as well as 3M. The big advantage of painters tape vs. other types of tape is the lack of residue when the tape is removed.
We've found walmart.com and amazon.com have some of the best prices on RV covers. Prices change (sometimes as much as 50%) every few weeks. We recently purchased a 35 ft. Classic Accessories Permapro TT cover from amazon.com for $200. Walmart wanted $290, while Campingworld wanted $570 for the same cover. Just a week prior, Amazon and walmart.com prices were exactly the same at $380. Sometimes walmart.com's prices come in lower than amazon.com---just a matter of timing. Campingworld's prices rarely change more than 10%--even when on sale. FWIW, items sold and shipped by walmart.com/amazon.com have a 90/30 day return policy, respectively.
Lastly, just wanted to mention Classic Accessories (Polypro, Polypro 3, Skyshield, Permapro, etc.) really stand behind their products. With a receipt and pics of any problem area they'll typically ship out a new replacement cover within a few days. We've done this twice with no issues.