If the insides of your trailer tires are worn, then its possible your boat axle is bent. Replacing the tires will only fix the problem for a short time. Down the road you will be in the same situation. Replace your axle or take the boat off and take the trailer to a weld shop and have them weld a angle stiffner across the axle.
I had a lowe boat & trailer with the same problem, the axle was not near heavy enough to support the load and bounce of the boat going down the road.
Ted
Neither snow nor rain nor heat nor gloom of night will keep me from crappie fishing!
2010 Lake of the Ozarks Super Slab Champion