I have tossed this question to my son. He works at Magic City Ford in Roanoke and in the same building as the Ford Truck Fleet Salesman. Only thing I can think of might be gearing (dont know if it affects payload).
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These two sites describe the same vehicle but significantly different payloads for slide-in campers:
2710# payload
http://www.ford.com/trucks/f150/specifications/payload/
1831# payload
http://www.ford.com/resources/ford/general/pdf/towingguides/13flrv&tt_f150.pdf
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On Edit: I did just notice that the first table talks of a 150# passenger in every seat. I dont see that information with the second table.
