Will, my unmounted spare came to me free when I replaced my tires, it came off the left rear outer dually...
I actually kept both the outers, store the other in the S&B basement.
When you "lose" a tire on the road, you call your road service company, coachnet or good sam or whoever... and you wait patiently in your nice coach on the side of the highway while the semi's scream past you at 75 mph...
And when road service gets there, they may not have the right size tire on the truck, and they may choose to charge you 3 times what whatever they have is worth, cause, after all, you are stuck on the side of the road with a 2 legged motorcoach...
Road service contracts cover the service, not the tire. I'd never leave home without my old unmounted spare. It ain't much, but it's free. It don't take too much space, and it will at least buy me some time.