Sounds like you may have a ball joint problem. I recently experienced the same excessive wear on the inside of one of my front tires. Diagnosis - bad lower ball joint. Jack the car up under the lower control arm at the tire(s) in question and check for excessive play in the tire (grab the tire and try to move it side-to-side and up and down). It shouldn't move much at all. I agree that MB's wear tires differently, but it sounds like you've got some excessively warn parts. Since my ball joint replacement, new tie rods, and new tires, everything is wearing and driving normally.