I called my local fleet dealer and they said the F250HD has gvwr of ~8500 lbs. and curb weight is just under 6000lbs. This would leave 2000 lbs capacity after allowing 500 lbs for fuel (dual tanks full) and passenger.
The guy said they use an F250HD to haul 500 gallons of water in the bed and it doesn't seem to be a problem. Doesn't even weigh down the truck. I wouldn't replicate their "use" of their vehicle, but I must say it is impressive to hear since the water alone would weigh 2 tons!
That diesel forum is pretty interesting. I wonder if they have other related forums and how I could get to an index of those forums??? Thanks.