First you got to put the truck on a scale so you know the weight of the front and rear separate (plus do it again with the trailer hooked up)
There used to be a graph on the glove box. It has listings for all tire sizes that come on the truck. You look up your tire size, then move across to the weight on the tires and read the PSI.
I have heard of others putting a chalk mark across the tire. Move the truck so the tire moved a couple revolutions. If the chalk mark is missing in the center your using to much pressure. If the mark is missing on the outsides your running to low pressure. If mark shows even wear your OK.
On my 98 2500 I used to have to run 25psi in the rear to get even wear when empty. (book said something like 45 empty, in 10K miles I could see the center of the tire was wearing out). I raised heck with dodge over this trying to get them to help replacing tires. They said "when we say empty, we really mean full of tools because its a work truck". In my book Empty means Empty!
Another benefit of lowering PSI is it stops wheel hop when going around a corner and you hit bumps with the fuel on.
Now towing with the SRW used to run 80 rear and 55 front.
In my dually the tire chart don't go low enough to say what PSI I should be running empty. There is something like only 750 lbs weight on each tire.
On another note, I have heard people say "put the pressure higher to save fuel". Sure you might save some fuel but your wearing out the center of the tire faster. I bet your fuel savings are no where near the price of replacing tires more often.
Everybody here has a different PSI but everybody here has a different load in the truck.