Happy Camper -
Don mentioned that you have an RTS. The original factory tires on your coach were 12R22.5s. This tire size was chosen for a vehicle in revenue service.
For RV use, many busnuts have gone to the smaller 11R22.5 size, for various reasons - mostly because the 11s are less expensive than the 12s.
Generally speaking, unless your interior is outfitted with marble floors and granite counter tops (i.e. HEAVY), this isn't a problem.
As Sean said, however, it does mess with your speedometer readings, fuel mileage and ground clearance. These may, or may not, be a concern to you.
The bus/trucking industry is slowly converting to metric sizing, just like automobiles use. It will be awhile, so a lot of the old numbers are still current and useful, but that's changing.
Most newer coaches are coming with 315/80R22.5 tires, which, as has already been mentioned, is approximately the same size as your 12R22.5s. Not enough difference to really quibble over.HOWEVER -
Be aware that bus tires are made in a couple of different configurations - one for regional or city use, and one for long-haul or highway. The difference is in the sidewalls primarily - those designed for city/regional use generally have heavier sidewalls, because transit bus operators thoroughly believe that curbs are part of the braking system when pulling into a bus stop.
City/regional tires also have a lower speed rating than highway tires, usually 50 mph vs 70-75 mph.
So be careful when shopping for tires - do your homework!!
FWIW & HTH. . .