I got seven years out of my last set of Goodyear Marathons on our 8500~lb 5th wheel before I noticed a bulge in the tread. They had maybe 4k miles on them and I kept them covered. I think that was pretty good life out of any tire that sits for weeks at a time.
Out of necessity, I bought five Towmax E rated 15's. At two years and maybe 2k miles, they're still doing well.
we just ordered a Jayco 28RLS 8 weeks ago. I'm sure it will come with some obscure name Chinese tires. I'll check them often and run them 3-4 years and replace them with either Marathons or Carlisles.
Anyone tried Primewell ST E rated 15's yet sold at Firestone/Bridgestone outlets? I believe Primewell is their Chinese offering for cars, trucks and trailers. They look pretty meaty.