This is not a bad solution, but do please secure the tire to the motorhome somehow or another. It may stay up there very well under all ordinary circumstances, but it may only takes one hellacious bump to send it flying, and I would not at all care to be the unlucky soul driving behind should that happen.
Your gasser must ride ruffer than my DP. I've never gone negative gravity which is what it would take to raise the tire up 8" to clear the cover over the vent. Rolling a unmouted spare off the roof is easy enough and without air in it, it does not bounce.