Didn't know about ageing issue when we bought our 36' Foretravel and put 10,000 miles on it, found out the front tires were 10 years old and the rear duals were 7 years old. Small cracks were visible around all the raised lettering on the tires. Tires were covered whenever parked for more than a couple days.
So-o-o, bought all new Michelin tires this spring and saved $78 per tire (235/80R/22.5 XZAs) Rides great, handles great, and tires are covered almost every stop for two days or more.
I think the reason our tires lasted so long was being covered and the fact that our coach only weighs 22,000# total, loaded.