Under Inflation - Wear on both edges: 

Under inflation of a tire reduces its tread life by increasing the tread wear on its outside edges, or shoulders. It also generates excessive heat which reduces tire toughness. Finally, it reduces fuel economy through increased rolling resistance because soft tires make your trailer and vehicle work harder. Abnormal tire wear may also be caused by misalignment.

Over Inflation - Wear in center: 

When a tire is over inflated, the center of the tread bears most of the load and wears out faster than the outside edges. Uneven wear reduces the useful life of a tire. 

Cupping - Cups or dips in the tread:

Cupping (also called dipping) is most common on front tires, although rear tires can cup as well. It may be a sign that wheels are out of balance, bearings are loose or that suspension parts are worn out.

Inside or Outside Wear: 

If the edges of your tire tread take on a sawtooth or feathered appearance, it's because of erratic scrubbing against the road. The solution is an alignment correction.

If the inside of tire is smooth or shows signs of excessive wearing  it is likely the result of overloading the trailer.