I reaplaced six Michelin tires on our Tioga 26Q, two of them were only 4 years old but already had medium depth sidewall cracks. I hadn't been covering tires but I do now. Tires that are over 5 years old should be inspected for sidewall cracks and replaced if cracks are significant. I have read that RV's need to be driven frequently at highway speeds to circulate "lubricant" in the rubber to get best tire life. When a newbie to RV's, I had overaged rear dual tire come apart and the flailing steel belt damaging holding tanks etc. Why risk it?
