Ford F-350 Towing Spec

The Ford system is identical to Reese stuff, so that's probably who makes it. Reese fifth wheel hitches that fit the under mount (Elite series) come in 18K and 24K ratings. That 18K spec likely comes from the Reese rating of the lighter hitch although I believe the under bed mount is rated for 24K. So, the answer to the OP's question IMHO is that the 18K spec refers only to the fifth wheel hitch. Ford will have a lower tow rating that includes the entire truck: frame, wheels, tires, engine, transmission, axle, etc. The tow rating will be lower for an SRW. In my experience I have exceeded GVWR or axle rating well before exceeding the tow rating. In fact our 12,500 lb. RV Fifth wheel puts us right at GVWR with our 2012 F-350 SRW, yet we are under the tow rating by 2,000 lb. Note: I believe Ford had not yet subscribed to the standards for determining tow ratings in 2012. If the OP is concerned about exceeding ratings then there are several such as Gross Vehicle Weight Rating, Gross Combined Weight Rating, Rear Axle Weight Rating. Even with a well balanced trailer, 20K is going to exceed the Combined Weight Rating.