Some rigs no longer come with a spare and the larger sizes would take up a lot of space. In addition their weight precludes most RVers from changing them. But of course having at least the tire sans rim would be very helpful if needed. Kidoo posted some good ideas. I would not want to lose the storage space on our extended trips.
Here is one persons approach to carrying a spare. This is a significant welding and cover project. And this carrier may not be the best idea with a rear radiator DP, but it gives the owner options if there is a tire problem.
Wish I had this tire carrier but I'm thinking about adding a bicycle carrier instead.