I carry a Host Mammoth on a 2001 F250 4x4 Extended Cab, Long Bed, with 7.3/6spd.
The truck came from the factory with the heavy duty suspension, which is overloads. I've install Bilstein HD shocks, a hellwig rear sway bar, and air bags, plus 19.5" Vision rims with G rated tires. The truck has been hauling a camper (previous 1998 Lance 9.6' 500) and the Mammoth for almost 100k miles.
It does pretty good, the front ball joints are going so it's started to wander, and the brakes got smoked coming hwy4 to monitor pass (that's a tight, twisty road!)
I personally would call the local junk yards, and see if you can find the overload pads, and replace the rear spring with the beefier HD suspension packs and stable loads. If it still sags too much for your taste (and you'll know it because everyone will be flashing their lights at you at night) then go get air bags.
Also install a Hellwig Big Wig or equal in the rear, HD shocks, and go weigh it before settling on tires and rims. Remember the factory rims are NOT rated higher than about 3415lbs per rim, and make sure if you buy rims that they are rated to carry the load; this is why the Vision 19.5's rims are so popular for heavy hauling.
