Michelin actually recommends a thorough inspection inside & out every year starting at age 5. That's probably their liability lawyers talking, since their engineers seem to think 10 years is reasonable. One of the concerns is that tire damage may be apparent only when viewed from the inside, so a superficially good-looking tire might still be problematic. Not clear what the odds of that are, though.
The rationale for 7 years is that the greater the age, the greater the risk of a blow-out, and a blow out on an RV frequently does body damage and/or tears up wires & plumbing. That makes a tire failure more than just inconvenient.
I would not replace a tire in apparent good condition simply because it was 4-6 years old, but I would start planning for the expensive purchase. I might run it 8 years if all still looks well at age 7. However, that assumes a discerning eye for tire condition. Many owners only know a tire as that round black thing on each corner...
