If your tires are wearing out on the inside, the camber built into your axle is probably bad. Take a 4 to 6 foot straight edge, and hold it against the bottom of the axle. It should lay perfectly flat, or have a slight gap in the center. If not, put the straight edge on the top of the axle. If you see a gap in the center on top, your axle has lost it's camber. A frame or alignment shop can put the camber back in the axle, but my personal opinion would be to replace the axle with a heavier rated one. Probably 7 times out of ten, trailers sold with boat packages are too light once boats are rigged and fishing gear loaded. This link tells about the camber or "bow" built into the axle.
http://www.easternmarine.com/3500-lb-Round-Tube-Trailer-Axles/