Understandably heat/UV affects tires and will cause dry rot but the real issue is sitting. Tires that are exercised regularly continually to stay flexible and release oils and compounds that resist dry rotting. I would love to see any evidence to support tire covers on tires just sitting. I doubt they have any real benefit to tires that are driven on regularly. So if your RV sits for long periods of time in the sun, they will probably offer a minimal benefit. How much is an elusive question. If you drive your rv frequently it's probably a waste of money.