Tire size conversion
I'll bet that if you check the tire manufacturers' published specifications for revolutions per mile, you will find that 275/70R22.5, 265/75R22.5, and 255/80R22.5 are functionally equivalent sizes. They are available in several different load ranges, so you have to be sure that the tire you choose can safely carry the load of your motorhome.
I'll bet that you also find that the various tire calculators you find on websites, many of which are unauthorized (stolen) copies of the miata.net tire calculator, are demonstrably inaccurate when compared to actual tire data. Despite this being discussed ad nauseam on many forums, and many efforts to get the writer of the software to fix it, the errors have not been corrected and people still link to them as if they are authoritative references.
The tire manufacturer's data is the authoritative reference. I would use it.
2000 Holiday Rambler Endeavor 40PBD
Freghtliner XC, CAT 3126B