Good advice so far. I would add the following to clarify weights a bit. The Ford E450 chassis (most common on a C) has a GVWR of 14500 pounds. The RV manufacturers the house part of the RV, so that the weight of the house can vary greatly. Generally, the longer the Class C the more it weighs.
Further, the weight of the house part is increased by the addition of certain options like leveling jacks, slide outs, and after-market goodies. The weight of fluids like gas and fresh water is also added to house weight. Subtract these weights from the GVWR, and you get the OCCC Occupant and Cargo Carrying Capacity, that is the amount of weight of people, food, pets, and stuff you can carry.
The GCWR is the total weight of the RV and tow vehicle weight that the chassis can carry. On the Ford E450 it is 22000 pounds (I believe). So the fully loaded RV, the tow rigging, and the tow vehicle should not exceed this weight.
If you consider a Super C unit, remember certain state like PA where I live will require you to get a Commerical Drivers License if the GVWR is 25000 pounds or more.
Good Luck with your search.