Max pressure for your truck is when it's fully loaded. On your trailer, it's loaded. If there is no placard on the trailer, then use what it says on the tires. Mine are 50 psi and I put about 46-48 lbs in em.
On my pickup I usually keep the fronts well inflated as the motor weighs them down. Rear is where you can vary depending on load. Empty, you probably don't want max psi in the rear anyway, it will bounce around.