I installed a set of Firestone air bags on my 2000 F-150. While they won't make it into an F-250, it sure feels more like an F-200.
While I haven't had the need to tow anything yet, I have completely filled the bed with rubble and trash for a few dump runs, and where the bed used to squat down, it stays level.
The ride on mine has improved significantly at anywhere between 15-30 psi. Loaded or not.
The real test comes this fall when I put my "redneck hunting lodge" (toyhauler) on the truck and tow it up to deer camp. Last year there was a lot of sag, even with the load leveling hitch. Everything I have seen so far leads me to believe it is going to be perfect.
P.S. I cheaped out on the on-board air compressor -- I just got a 12 V job at the auto parts store for $20. Needed an extension cord to make it reach, another $12. Sure, I have to stop to change the pressure, but I don't expect to do it more than a few times a year.