you get Free Roadside "service" towing etc however you will pay for a tire at the time.That will be changed roadside
Motorhomes (especially ones with 22.5 wheels) do not come with spare tires. Since you are now essentially in the "heavy Truck" category a commercial Truck service will come to you with the tire.Most large trucks do not carry a spare.
As stated you can purchase a tire and carry it with you if you have room. You will save a few bucks on the tire .. maybe a hundred dollars! ?
My opinion is However the chance you will have a flat is rare and the tire you purchase will age out the same as those in use. and technically will need to be replaced when the others are at 6-7 years. Thus carrying around a 4 -5 + hundred dollar ,up front cost, tire that most likely will never be used to "save a hundred bucks ...IF you ever need it
.I quess as a commercial driver I never have understood the economy of that choice.
as to if Your size tire is common call a few "Commercial" heavy truck tire places that do Towing ( the guys you will meet roadside,and thats not your corner tire store) to see if they carry it in stock and the cost.
Only you can decide whats right for you.