While I agree whole heartedly with Clark and Jim about getting the trailer weighed so that you know what weight is on each tire, there is one other factor that is frequently overlooked on tires and that is the stress that is put on them in side to side movement which we get when backing into a site. Load ratings are determined by the vertical load placed on the tire. The horizontal load from turning can be, and often is, significantly higher and can damage or weaken the sidewall strength. With the air pressure set at the tire manufacturers recommended pressure, this reduces the chance of damage to the tire sidewalls from this stress factor.
ddj, when my Grand Canyon came from the factory, it had 75 psi + or - a couple in each tire. Before I left the dealer, they had aired them to 110 psi. I, so far, haven't had a problem with my tires and I'm somewhere around 10,000 miles and they are the original Trail America, made in China tires. Today, my trailer gross weight is slightly over 16000 pounds and is as balanced as I can get it. My pin weight is about 3200 pounds. I hope this helps.
John