I have a 22FT Class C. I opted for a 4"x 4" Bumper Spare tire mount. I couldn't find one made for a dually wheel, so I got one for made for a trailer wheel and modified it. See
https://www.amazon.com/gp/product/B011RIT82O/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1
I cut off the "strap" and drilled holes in the post and mounted the wheel directly to the post. A dually wheel would not fit on the strap where a trailer wheel will. I don't plan on changing a flat myself, I want roadside assistance to be able to change the flat easily. The Ford E350 Chassis has dually wheels and the one spare will fit in any of the 6 positions.