The tires I had on the MH started going at 6 years. They were NOT covered and used only a few times a year for the last several years prior to the blowout. The tire that went was the one that received the most sun. Interestingly the spare was 16 years old (a Michelin) and was fine for several hundreds of miles after it was put in use. It was stored in an enclosed area in the MH. I replaced it when I replaced all the tires, but I'll bet that it would have lasted longer if kept in the enclosure.
I cover the tires now, and get the MH out at least every other month to condition the tires.