Payload on used truck?? -- can't weigh beforehand

Ditto on dually, although weight distribution can make significant change.
My 11.5' camper is rated 3800lb by the factory, yet I am only 300 lb from maxing rear axle on dually and I really did not load it heavy.
Unfortunately campers do not allow you using front axle capacity, unless you move spare tire and propane tanks onto front bumper.