If you decide to use Michelin tires, check out major brand tire store prices on the internet, print out page with price, balanced, installed and other charges. Call local tire shops and get estimates for six (6) tires. They will usually try to match or beat internet dealer price. Discount Tires is a good source, Allen tires is another. I don't think that Costco mounts RV tires.
Have long solid metal custom formed valve stems installed while you are having new tires installed. These things make checking tire pressures easy with beauty disks installed and don't leak. Don't use cheap screw-on valve extenders, they can unscrew, leak and cause tire failure. Google Borg or Tire Man.