The only way to know the true payload of a vehicle is to put it on the scales and work back from there with the GVWR of the truck.
On a new F150 Ford advertises the payload of a 5.0 litre crew cab 5.5 foot box to be 2050 lbs. Who know what the heck that did or did not include (driver/gas/bumpers/tires, etc....).
The dealer I eventually purchased from was able to take one I was looking at and put it on the scales. Working back from there it had just about 2000 lbs of payload after factoring in a full tank of gas and nothing else.
I have used the rule of thumb that the payload advertised does not include anything. So if it says 1800 lbs including driver, I would not include the driver and just start with 1800 lbs and work back from that, expecting that the 1800 lbs is on the generous side.
Same with trailers and tongue weights. Just go with the GVWR of the trailer and usually add the weight of a battery and propane to the tongue.
It truly boggles the mind how these companies come up with their weights...
