I would have the bushings checked in the front end. MIne had to be replaced at around 70K miles and eight years old. The alignment also has a lot of negative camber, around minus ten degrees, which aides in handling, however this will contribute to wear on the inside of the tire. It took me a couple different shops to get the alignment correct. Since only the toe setting is adjustable technicians will try to compensate the toe to achieve a straight steering wheel or even pulling to one side. This can have a negative affect. I had the camber/caster kit installed so the camber and caster could be adjusted. I don't believe that the alignment service at a normal tire center may not have the expertise to understand what is necessary in a complicated front end such as in a SL. I assume the front tires that are wearing unevenly. Twenty five thousand miles is about average tire life, varies from 20 to 30K. Have those bushings checked.