Also, keep in mind that your 'take-off's' may have some value.
We drove the first set of our previous motorhome tires for 6.5 yrs. and 57,000 miles. I bought new tires through the FMCA Michelin tire program and the tire shop gave me $50 each for my eight tires. I drove the motorhome another 1.5 yrs. before we sold it. The newer tires helped the value of coach.
Like everyone here, we ALL like to get every last drop of use out of our tires. We just don't know that exact date. But, what we do know is that when a tire blows it can get quite expensive.
To me, 7-7.5 yrs. is just a good time frame to change tires.
Safe travels,
MM.