at 28,000 miles, got a message to service DEF system. Dealer found shorted DEF tank heater. Replaced it. Replaced reservoir. Prior to this, I always got a few mpg drop during regen. After this, I lost about 3 mpg and am never aware of the regen. Back to dealer at 36,000, they state no error codes and everything is fine. Anything I can ask them to check to help get the mileage back where it was. Truck is stone stock. Help Please.