Start with real available payload
Use the payload rating for the exact truck, then subtract the weight of occupants, bed cargo, accessories and hitch hardware. What remains is the approximate payload available for trailer tongue or pin weight.
Convert remaining payload into a trailer estimate
For a conventional travel trailer, tongue weight is often planned as a percentage of loaded trailer weight. If 900 lb of payload remains for tongue weight and you are planning around 12%, dividing 900 by 0.12 gives an estimated 7,500 lb loaded trailer weight.
This is only one screening check. The final setup must also stay within tow rating, GVWR, GCWR, axle ratings, tire ratings and hitch ratings.
Fifth wheels use payload faster
Fifth-wheel pin weight is commonly a larger percentage of loaded trailer weight than conventional travel-trailer tongue weight. The hitch itself can also be heavy, so payload and rear axle capacity deserve close attention.
Do not calculate from dry trailer weight
Use expected loaded trailer weight, not brochure dry weight. Water, propane, batteries, food and camping gear can materially increase both trailer weight and the load transferred to the truck.
Next step
Use the TowAtlas Payload Calculator to model the load, then run Rig Check to compare the major ratings together. Before towing, work through the Pre-Tow Safety Checklist.