If your underinflated then the outside and inside edges will wear faster. If just the outside edge on all are wearing faster you may have too much camber. If only one is wearing on the outside then you may have an alignment problem.
The axle should have an upward curve in the center. When a axle is over loaded the tires will be tilting in at the top. Under perfect conditions you tires unloaded will tilt slightly to the outside at the top. Then when the trailer is loaded up they should straighten out and be straight up and down.