[Johnnyfry]

All tire manufacturers publish a load VS pressure chart. The pressure (cold) you put in depends on load. The best way to determine this is to get weighed. You need the weight on each tire so if you weigh the front axle you divide by 2 to get the individual tire load. Same with the rear. If you have a Dully assume that each tire is 1/4 the axle load. Same for the trailer. Knowing the load on the tires you can go to the chart and read the cold pressure you need. If you are close to the max lead on tires this will probably be the max pressure stamped. Also be aware that the max rating for a tire will be lower for each tire on a Dually axle.

Don't worry that the pressure goes up after driving even a short distance, this is normal.

I hope this helps.
John