Tires deform; they don't compress, so the air pressure remains the same regardless of how much weight you put on them.
Perhaps true - more or less, but the correct way to do it is to have your truck loaded as you would for the trip - including all passengers - and go and weigh all four corners if possible, and at least each axle, and use the tyre manufacturer's load-pressure tables to set the cold inflation pressure.
Rigs I have seen have shown tyre wear consistent with way too high pressures and this isn't good for tyre wear, handling, braking and especially ride comfort.
Once washboarding or rocky gravel is met, it is beneficial to drop the pressures to 80% AND drop the speed to at least 80% as well. The worse the surface, the lower the speed and the lower the pressure.