The world of tow vehicles would be a better place if

ford truck guy wrote:
How about a C-15/C-16 tucked under the hood of a F350.....

LOL! By the time they beefed up to hold the weight, added the engine, you would likely be over the class 3 weight.
Easier would be replace the cab on a class 8. But if you showed up at a CG towing a large camper some would get panties in a wad because the fender emblems where overloaded.