Load the tow vehicle (TV) with everybody and everything that will be in it when towing. Go to a truck stop that has a certified automated truck (CAT) scale, fill up with fuel, then weigh the wet and loaded TV. Subtract the weight of the wet and loaded TV from the GVWR of the TV. The answer is the payload capacity available for hitch weight.
If the wet and loaded TV did not include the head of your weight-distributing hitch, then subtract about 75 pounds from the payload capacity available for hitch weight to get payload capacity available for tongue weight.
Divide the payload capacity available for tongue weight by 0.13 and the answer is the max GVWR of any travel trailer you want to consider.
If the wet and loaded TV did not include the installed fifth wheel hitch, then subtract about 125 pounds from the payload capacity available for hitch weight to get payload capacity available for pin weight (weight of the kingpin on the hitch).
Divide the payload capacity available for pin weight by 0.18 and the answer is the max GVWR of any medium size fifth wheel RV trailer you want to consider.
Understand that those tow formulas will give you ball-park estimates, not exact numbers. Tongue weight of a wet and loaded TT varies from about 12% to about 15%, and I use the average of 13 percent. Pin weight varies from about 16% to about 20% for medium-size 5ers that are not luxury RV coaches, with the average of about 18%. Larger and more luxurious 5ers have a higher percentage of pin weight, but you cannot tow one of those with a 2500 without being overloaded.
Grumpy ole man with over 50 years towing experience. Now my heaviest trailer is a 7,000-pound enclosed cargo trailer, RV is a 5,600 pound Skyline Nomad Joey 196S, and my tow vehicle is a 2012 F-150 EcoBoost SuperCrew.