ST, I went through the same analysis last fall and ended up with an F350 pretty much exactly like yours. However, I do think the payload number on the F250 is an artificial limit driven by marketing rather than technical aspects. I understand that anyone using a truck in interstate commerce is subject to commercial driver license and rules if the truck has GVWR of greater than 10,000 lbs., and I'm guessing a large part of Fords target market for F250 is commercial. I've been through all the technical specs for axles, tires, springs, etc. and there is no apparent technical reason for there to be 1500 lbs. difference in the payload rating of F250 Vs. F350. However, to avoid any legal exposures due to "overloading" according to the nameplate, I went ahead and got the F350 as well. This did create a different problem for me, as it is now impossible to level my trailer when towing, because the bed is so high on the truck, and I have torsion axles on my Rockwood. Fortunately I have enough excess capacity on the trailer axles and tires, that even with the extra weight on the rear tires due to the out of level, I am still below the load specs, and it tows just fine. Also, I've checked the tire temperature and pressure rise when travelling and haven't seen any significant difference in front and rear tires.
2010 Rockwood 8280WS Fifth Wheel
2013 F350 Crew Cab SRW, Standard Bed PS Diesel
Reese Airborne Pin Box, Reese 16K Manual Slider
Camped 28 Days in 2012, 23 Days in 2013, 29 Days in 2014, 30 Days in 2015, 9 Days in 2016