"is the limitation of most Gas class A and C MHs just the hitch?"
Well, not really. Our chassis is rated for 26K lb. combined, 22K lb., gross, and has a 5K lb. hitch.
If we pull a 5K lb. towed, our gross chassis weight can be 21K lb. (if following manufacturer's limits). Which would mean we loose 1K lb. of cargo weight.
But, if you don't bother with the combined weight limit imposed on your chassis, and you upgrade your hitch, then it is possible. However, chances are you will stress the engine, frame, and transmission excessively over time. And then there is the problem of accidents and sue-happy people.