I have a 2014 GMC 3500 with 6.6 with 110k miles on it. Highway mileage is 20-22mpg when empty. I used to get 18-ish towing my 32 foot 5th wheel (14klbs) but now getting 9-12mpg. I had radiator and turbo replaced last year. Oil, fuel and air filters changed ahead of schedule....any thoughts? Transmission issue?