Also Trailer Life's Tow Guide lists my '02 F250 SD 7.3 diesel at 12,800 max 5th wheel towing and Ford lists its max gvwr @ 20,000 lbs. so I am still under the max @ 18,700.
True, but you ignored the fine print. The Trailer Life Tow Guide copies the Ford RV and Trailer Towing guide, which says your max 5th wheel trailer weight is 12,800 pounds, PROVIDED
you do not exceed either the GVWR or the GCWR of the truck.
In order to tow a 12,800 pound trailer with a GCWR of 20,000 pounds, your wet and loaded tow vehicle could weigh not more than 7,200 pounds. I'll bet you a Rib-Eye dinner than your truck weighs more than 7,200 pounds when wet and loaded with driver, full tank of diesel, and 5er hitch, along with an adult passenger, toolbox full of tools, cooler full of cool, and options such as a spray-in bedliner. Mine weighs around 8,000 before I back up to the 5er, and it's a 4x2.
If you can throw out tools and passengers and stuff to get the weight of the wet and loaded tow vehicle down to 7,200, that leaves a max hitch weight of 1,600 pounds before you bump into the GVWR of the truck. A 12,800 pound trailer with a hitch weight of 1,600 pounds is only 12.5 percent hitch weight. Most 5ers in the 12,000 pound weight class have a hitch weight of around 17 percent. So even if you get the weight of the tow vehicle down to 7,200 to stay below the GCWR of 20,000 pounds, you'll still be overloaded over the GVWR of the tow vehicle.
If you exceeed the 20,000 pound GCWR, you're overloaded. Or if you exceed the 8,800 pound GVWR of the tow vehicle, you're overloaded. With my 25-foot 8,000 pound 5er, my F-250 is overloaded, although my combined weight is only 16,000. So I'm pretty sure than anyone that tows a 5er that weighs more than 8,000 pounds with a '99-'04 F-250 diesel pickup is going to be overloaded.
You've got the CAT scale ticket. What does say?