Tires that are used regularly like car tires rarely need covers. This is due to volatile s in the rubber that migrate out to the surface over time. RV tires, show cars, etc. sit for long periods of time. The combination of sun and weather take their toll.
Keep your Rv tires fully inflated, take some weight off if practical and cover them from sun. Some tire sprays have UV guard chemicals but I doubt they would last more than a few days. If you have inside storage no cover needed.
I have my Raineers covered even though I intend to move up to Goodyear Endurance tires in the spring. As they are only two years old someone on Craigslist will want them.