Issues with DEF system. Fuel and DEF tank is filled up. Within 100 miles of towing the DEF system states DEF is empty and Speed will be limited within 500 miles. At the time the message appears on screen, fuel mileage goes from 13ish mpg to 8 mpg. Try to add DEF and only able to add close to a gallon of DEF. System has a hard time recalculating the amount of DEF in system.