Buying a used tire to carry as a spare?

X2 good idea, that way you know where the tire came from.

Why not get two new tires and mount them on the front. That way the steer tires are less likely to fail. The tires you take off the front can be your spares either mounted or unmounted as you prefer.
That would be the safest thing if you feel your tires are getting long in the tooth

I carry two mounted spares and rotate them into the mix every couple of years.

This is one of those topics that comes up now and again. there are 2 schools of thought to this. Ones that would not be caught dead without a spare and some that use RSA and let them bring and mount you a new spare. Pros and cons to both. I am on the latter side of the fence and feel carrying a spare I may never need is not only a waste of money but a waste of space. When I can call coachnet to bring me a new tire and have them mount it. But that's the direction I chose. I imagine that's the majority since new coaches no longer come with spares or easy places to put them. But if you want to buy an old tire and let it sit in the bin then I imagine you'd want to buy it from a reputable shop and have it checked out. But how long do you keep it before you swap it out? Sitting un-used in a bin or not - it's going to age and lose air. When it comes time to need it you may find that either it may not be up to the task, or that it only gets you far enough to buy a new tire anyway. How long do you tempt fate on your used spare before you replace it? In which case now you've bought 2 tires. Despite RSA taking longer in remote places than others, I've never heard of anyone stranded indefinitely. I mean if you have to call RSA to change your spare, only to probably have to buy a new tire anyway, why not just have RSA bring you a new tire and save yourself the money you spent on the spare? The risk, I assume you are trying to mitigate, is RSA not being able to find your specific tire. Are they that uncommon? I mean you're going to have to find one anyway in pretty short order. Just not sure what the upside is.

good idea ,check around and find the best you can, any tire is better than no tire.

I carry a mounted spare, a compressor and air tools. If I'm lucky, I'll be able to change it myself, but if not, then I've got a spare for the roadside assistance to use. Of course, on the two blowouts that I've had, the first took out both duals, so the spare was a moot point. We got them to bring two new tires. They changed both roadside. On the second, after checking the date of the spare, I decided to get them to bring another new tire. I now have a new spare, mounted and ready.

All that to say, there are many trains of thought on this issue. It's ultimately up to you, but I'd beware of keeping an older tire as a spare. If you absolutely wanted to, I might would consider one of the inner duals to be used as the spare when you change your tires. Mounted or unmounted, again is up to you. You'll save several pounds of weight by going unmounted and unless you're going to try the tire change yourself, the technician will be able to do it roadside with no problem.

Although I have a 5th wheel, I once stayed at a rest area and saw a guy with a Class A waiting for RSA for an hour. He said he had waited for 4 additional hours. He had a full spare tire in his cargo but couldn't lift it by himself. I helped him take the flat off and put the spare on. His was a new tire on the wheel. By helping him, he was able to get back on the road. I would suggest a full spare if possible so that when changed you are not rolling on a old tire and don't have to worry about getting it changed as soon as possible; only getting a new spare on your wheel. But, cost and weight are involved. Good luck

I carry an unmounted spare of the same type as my other tires. Altho heavy, I can take it out of the basement compartment or move it around myself. With a rim it may be too heavy for some to handle. Only you know your own weight limitations.
Changing an unmounted tire only takes a few minutes for a tire guy with a pair of tire irons. Just a little longer than it will take for him to install a mounted tire.