Cliff, many factors will influence the life of tires. A tire that is left sitting for many months at a time will have the polymers that help reduce aging due to sun and ozone dry out at the outer layer. Not good for tire life. Keeping a coach where the tires are not subjected to the sun most of the time is beneficial to tire life. Keeping a coach in an area less subject to ozone (long way away from major metropolitan areas) will increase tire life. Avoiding road hazards and curbs will increase tire life. Driving fast in hot weather will decrease tire life. Driving faster than the tire is rated will decrease tire life. Overloading tires will decrease tire life.
That said, all rubber produces will age. The older they get and the more abuse they encounter the more likely they will fail catastrophically. The 5-7 years is a rule of thumb but the question you need to ask yourself is "Am I feeling lucky?" Running on old tires and being without insurance are somewhat similar. As long as you don't have a problem, you save money. If you do have a problem, the out of pocket cost and the extra inconvenience (and safety in the case of rotted tires) can be really high.
If you don't want to just buy tires based on a rule of thumb, take the coach to a truck tire dealer with some scruples and ask them to un-mount them and give you an assessment of their condition. If they look good inside and out and are pliable, keep them another year. The money you spend for the annual inspection is a lot less than buying new tires.
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