5 years is the "industry standard" for tires,
What industry would that be? The tire makers mostly say 10 years of useful life, but some tire dealers (Discount Tire, for example) are now saying 6 or 7. GM, Ford, Chrysler & multiple European car makers are saying 6 years, but that's for new tires stored in warehouses. The RV industry, whether RVIA, the manufacturers or dealers seem to be silent on this subject.
All the experts, whether tire maker, tire dealer, or various industry gurus, say that the useful life is unpredictable once the tire is mounted on any vehicle. In addition to tread wear, factors such as climate, inflation, chemicals in the air, and the surface it sits on all play significant roles in tire life.
Anecdotal evidence here and at other RVer websites suggests that the hefty 19.5 and 22.5" motorhome tires will probably
safely last at least 7 years. The smaller 16" motorhome tires are probably a couple years less, perhaps because they are stressed more.. Trailer tires don't seem to last as long and size is a factor there too, but 3-5 years seems common.
Tire Rack has published what I feel is a well-balanced discussion on tire life. See
https://www.tirerack.com/tires/tire...D_BwE:G:s&s_kwcid=AL!3756!3!386427243778!b!!g!!