Our Bounder was built in October of 2006 - 5 of the tires had a date code of 2806, while one tire, the driver's side front, had a date code of 3405. They had just over 16K miles on them. During a pre-trip inspection I saw significant sidewall cracking on the driver's side front tire. Basically, everywhere there was lettering on the sidewall had cracking at the edge of the lettering. I just replaced all 6 tires through the FMCA program and saved $100 per tire over the best price I could find on the tires locally.
Unless you're on the road constantly, mileage means little when it comes to RV tires - check the date codes and, as Michelin states, after 5 years have them checked annually be a competent tire guy. Some people can get 10 years out of them; others are lucky to get 7 years. Covering the tires helps, but unless you can keep the atmosphere away from both sides of the tire the ozone still works its magic on the rubber.