If by 18K-pound Class A chassis you mean the Ford F53, a problem you will likely encounter is that tires are larger in diameter than what Ford specifications suggest will fit between frame rails; and if mounted below frame rails, somewhere behind gasoline tank, you "may" compromise ground clearance a bit much.
Also, that size tire/wheel combination (19.5") is heavy so moving it around, even on ground, would take a lot of effort.
These are reasons we probably don't see that design used often, so not much demand for such carriers. Only carriers I've seen for that size wheel/tire are hitch mounted, hanging off rear wall, or inside a large storage compartment.
A forum member here had an older motorhome designed by manufacturer with a spare tire carrier in back behind bumper. I'd guess it would have to be integrated into the rear chassis extension to achieve clearance.
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