Unfortunately that size is unique to Michelin. If you want to stay with the size a tire dealer should be able to order them rather than an RV service center.
What else to use? I favor the secondary brands such as Sumitomo, Kelley or Toyo over Goodyear or Michelin. 255/75R x 22.5 is pretty close but slightly taller and wider while 245/75 might be a closer fit. Shop around and use this size comparison to see what the differences are.
Wheel well clearance and adequate separation for the rear dullys can be critical.