Usually if the inside of the tires are wearing, you are over weight, or the axle has lost the factory has lost the camber they put in at the factory. If you put a straight edge across the bottom of the axle, you should see a gap in the middle. My one year old trailer wore the inside of the tires off, and when I put a straight edge on the axle, the gap was on the top, not the bottom. I just replaced the axle. Here's something from a trailer parts supplier.
CAMBER: Axles are cambered or 'road set' so there is a bow until weight is put on the axle. All cambered axles should be mounted with the center bow up.