I think car manufacturer's put recommendations in owner's manual's more to CYA on legal reasoning as much as wear and tear. Remember the issue with Firestones some years back if I recall on Fords and all the law suits. About five years ago I bought a 1977 Kawasaki from an acquaintance with only 3200 original miles on it. First think I commented about was the tires were still good. The guy, who is an industrial engineer, told me that's because the bike was stored away from any electrical sources, what ever that meant. Drove it two seasons until I noticed the side walls starting to crack and I replaced them with Dunlop's. Another acquaintance (I have no friends!) owns a 1986 Winnebago and is still driving it with the original rubber. For me its due diligence on keeping them inflated, away from petroleum products and covered during storage. I inspect them before every outing. I have heard on forums only about some that have found the tires cracked on the inside with out showing any exterior cracking. I am curious though that most if not all RV tires, at least 22.5 inch are built much like truck tires. That is why they are re-groveable the same as truck tires. Truckers don't chuck their tires every 6-7 years. The run the heck out of the threads and have them re-groved. Side walls are fine.
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