Mileage is typically stored in the vehicle's on-board computer. In the 2015 Silverado, this information is likely stored in the instrument cluster module. This module houses all of the vehicle's gauges and warning lights, so it would make sense for mileage to be stored here as well. If you're interested in finding out exactly where your mileage is being tracked, you can consult your Owner's Manual or contact a Chevrolet dealer.