If you do not mind paying TOP DOLLAR for a tire replacement, regardless of size, then don't carry a spare. The size does not matter when you are stuck somewhere where you only have a choice of ONE tire dealer. They will charge you whatever they want and you have no choice but to pay, even if the new tire is not a match to your other tires because it is a different brand.
If you carry your own spare, any tire mechanic can mount your new spare on to your coach in a matter of minutes, even if it means removing the bad tire and mounting your unmounted spare.
I carry an unmounted spare because it is a lot lighter and I know that it matches all of my other tires.