"For towing a big trailer behind a motorhome, a super C / "toterhome" is probably the best option. "
Agreed! Not having those 22.5" wheels all around mounted on a true long haul truck chassis, (like a Freightliner for example) IMO, are the weak points. How does changing the rear end (carrier/ring gear size and ratio) increase Fords 16" tire/wheel load capacity?
https://www.ford.com/commercial-trucks/e-series-cutaway/models/
https://www.fcccrv.com/chassis/s2rv/
Bigfoot clearly extended Fords E-450 cutaway chassis 176" wheelbase to 200" on the MH29SL. That will help with the stability but GCWR is the issue here. Most +-30' E-450 Class C's with slides are at or close to max GVWR when they roll out the factory door, . . . . dry.
Chum lee