Per Ford:
"Gross Axle Weight Rating is determined by the minimum component of the axle system (axles, computer-selected springs, wheels, tires) of a specific vehicle. Front and rear GAWR's wiill, in all cases, sum to a number equal to or greater than the GVWR for the particular vehicle. Maximum loaded vehicle (including passengers, equipment and payload) cannot exceed the GVW rating or GAWR (front or rear)."
The AAM axles and wheel bearings are the same for the GM and Ram trucks and rated at 10,900 lbs. so the payload reduction is based primarily on the rims and tires provided at the factory. All the manufacturers also subtract 150 lbs. per passenger seat from the payload rating which is why you will find that the trucks with a regular cab have the highest load rating.
I bought a 2500 as I wanted SRW and the GM extended cab and that was not available in a 3500 model. Also most of the 3500 trucks on the dealers' lots were 2WD and DRW and I wanted 4WD and SRW. I spent $450 to add Supersprings and turn my 2500 Duramax into a 3500 Duramax equivalent. The ONLY difference between the 3500 and 2500 Duramax was the extra leaf pack on the 1-ton and the availability of DRW.
For a camper my first choice would be a GM or Ram truck to get the much stiffer frames these two manufacturers provide. GM upgraded its truck frames in 2011 for both 2500 and 3500 and Ram is doing the same as of 2014 (in 2013 only the 1-ton Ram trucks got the upgraded frame). With DRW the frame flexing is even more of a problem and I would not choose Ford but pick between the other two.