After a few years, the oils in the rubber drys out and the tire no longer flexs or it rots. Either way, about 7 years is the limit to be safe. On RV's, mileage is seldom the culprit. You trade them in and Manuel takes them , mounts them on his truck and runs them another 100,000 miles without a problem. On the RV, you run them another 1,000 miles and they blow and take $100K worth of fiberglass with them.
If you're unlucky enough to have Goodyear GY 670's on your steer axle, then it's a whole new problem.