Fifth wheel Towing Help

While planning on an upgrade soon, weight ratings have me baffled. I have plugged in the numbers on some weight calculators and they come up short of what I can tow including the trailer I am towing now!!! But, it gets better, the weight ratings show I am fine towing what I am now and the scales show me way in the good also. So here is my dilemma. Tow vehicle is a gasoline 2010 GMC Sierra 4X4 extended cab regular or short bed manufacture GVWR of 9900, 3:73 ear ratio. I am towing a travel trailer at 6400 lbs now without any issues. So the plan to upgrade to a fifth wheel with a GVWR of 13900 and hitch weith at 1955. Manufacture shows a GCVWR of tow vehicle at 16000. Going through the checks and balances of different calculators and using all the info I have I come up short of towing the new fifth wheel. Now the question is how come everyone is pulling a large fifth wheel with the weight ratings that don't work or not within spec? I am not one to ignore the specs, but just about ready too unless I by a 2 ton truck! I am sure my truck will handle the weight etc, tow ratings jump with different gear ratio and / or diesel so the truck is the same, just mechanical difference so fuel mileage will suffer is the main issue only? Am I crazy or what? Honest input will be helpful. I will get with some folks at PD commercial enforcement to help as well. Your input is welcome and I thank you in advance.

You need to tell us your exact truck.. meaning is it a 1500 or 2500? What bed and cab??

Then you need to tell us what your scale weights are of the truck only...

Then you need to tell us the axle ratings of your truck ( Inside door tag)

After we get that we will know your payload and be able to tell you what you can tow..

All other answers without that info would all be assumption period...