If you google 'tires wearing on inside on trailer" you'll find plenty of info. I'm betting your axle has lost its bow or just too light to start with. I have had that before. New heavier axle fixed it. Everything has to be right to get even wear. I have a small crappie jon was doing that. I custom made an axle for it and it has no bow...but I guess I made it so heavy it doesn't flex cause tires don't hardly wear at all now.