What does the sticker say on the van itself - that statement by Ford was in reference to an off the assembly line E-350 van, not one with a Class 'C' motorhome body on the E350 Chassis. Also, the 'up to' part of the statement means that when properly configured, it has that capability. Somewhere on your Class 'C' there should be a sticker put on by the coach mfg that will give you the rated capacities 'after' they added all their stuff, and I doubt very seriously, that it is anywhere near the GCWR of 20,000Lbs, and the relatively 'low' curb weight is nowhere near what an empty E350 weighs.
Larry