As Ivylog says, look for the Federal Tire Placard near the driver's seat. Use the pressures listed there, NOT the max pressures listed on the tire sidewall. Keep in mind that the Placard assumes that you are at your max GAWRs. You should never run a higher pressure than that, because you should never load your rig heavier than that.
As soon as is practical, get your rig weighed, 4-corner is best, but front/rear will suffice. Then use the pressures that the tire manufacturer recommends based on the actual weights.