Can't you have it mounted in the rear up in between the frame rails and between the gas tank and the rear bumper?
That's where ours came mounted, stock ... I assume Ford delivered the bare chassis to the motorhome manufacturer with the spare mounted that way from the factory.
This location gets the spare up out of the way and not so far to the back as to be bounching around as much on a long lever arm. It also looks cleaner to not have the spare tacked on the back coach wall. I does make it harder to get at it when needed, however.