Good advise from thewiz and browzin.
Weight the rig, both ends, as close to trip ready weight conditions as you can. Refer to the load charts. These inflation figures are the MIN pressure to support the weight. Since the listed pressure is less than the pressure molded on the side of the tire on my rig, I add a couple psi just in case we gain a few pounds while out on the road.
Oh, when you weigh the rig, it's best to weigh all 4 corners. Any inbalance side to side, inflate to the needed pressure of the heavier side. All tires on a given axle need to be inflated the same.