Hi,
It depends. On the manufacture and the way they extended the frame behind the factory framework. On my 1997 there is a 18" extension installed by Fleetwood. I have a 30' model with a 190" wheelbase. Some of Fleetwoods models that year had the 190" Wheelbase with a tag axle on a 38' motorhome. That would be a 9 or 10 foot long extension behind the factory frame work.
In 1997, Fleetwood rated all of their motorhomes to tow 3,500 pounds, no matter how long the extension was. By 2000, Ford was producing 18,000 and 20,000 GVWR F-53 chassis with 19.5" rims, good for much more weight than my 97 with 16" rims designed for pickups and passenger vans. So they started rating the Fleetwoods for about 5,000 pounds on the gas models. They also had more powerful engines and better - stronger transmissions.
I am not sure what manufactures made a RV with a 7,500 pound tow rating, giving it an extra 750 pounds of cargo capacity to carry the heavy hitch weight found on a car hauler.
I know that the Alpine diesel pushers had a 10,000 pound hitch bolted to their frames, and have a 400 HP diesel engine to power it up and down the mountains of Washington where they are built.
You might also want to look at the Supernova by Gulfstream. Those that where built on a International chassis are rated to tow 10,000 pounds, and have a big diesel engine to power it. It is a class C built on a 25,000 pound truck chassis. One like used in the larger U-Haul trucks.
Other manufactures have used the Ford F-550 truck to make class C motorhomes on it's 19,500 pound capacity chassis.
The E-450 chassis is limited to only 14,050 pounds. Even rarer is the 2005 E-550 chassis with 19.5" rims, and a much higher GVWR. However it did not take off, and Ford stopped making them after just one model year. I saw a tile floor installer who had a E-550 truck with a 20' body on the back of it. It was able to tow a lot of weight too.
Remember that your intended trailer will have about 800 - 1,000 pounds of hitch weight. So when you look at the typical class C, with only about 1,500 pounds of cargo capacity (including the water tanks, and everything else) you do not have much capacity to carry your camping gear and 800 pounds of hitch weight. Take the RV to a scale. If you find a 12,000 pound motorhome on a 14,050 GVWR chassis, that is the rare one, probably without a slide out.
Good luck on your search.
Fred.