We have a 2000 F350 SRW longbox, with a Bigfoot 1500
We run the stock Ford 16″ Alcoa rims with the Goodyear Wranglers (I forget what size they are)
We looked into the safest low pressure to run the tires for our weight, and printed a little label for the inside of the drivers door jam for max low and regular pressure and when we air-down I just read the number, I can’t off the top of my head recall that pressure.
We don’t “wheel” our rig but we do spend some time on forest service roads, mostly in the Western USA, so we air down mostly for comfort or for traction if it’s sandy. It makes it a bit nicer especially on corrugated/washboard roads.
We don’t go very fast when we are aired down, but then we subscribe to the school of thought of off road driving by going just quick enough to not get stuck.