6.6 serious drop in mileage while towing

[TNCannons]

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?

[BigBadSierra]

Check the Turbo plumbing for an air leak. 

 

Do you feel like it has lost power and is revving higher to do the same job.

 

I assume there are no abnormal weather conditions involved it's amazing how bad your millage can drop just from added wind drag.

 

 

[TNCannons]

GMC replaced turbo but mileage stayed same. No leaks noticed. Idle does seem to be a bit high when in park but no other issues. Power is great, truck runs extremely well. I live in east TN at about 900 foot elevation so usually warm, no extremes in weather, just lots of rain. Where would I see turbo leaks? Appreciate your help and thoughts!

[BigBadSierra]

The leak may only occur under heavy load that's all I'm saying.. When the system is under higher pressure. 

 

It's worth a check.

[2017Darkness]

EGR is probably getting sooted up. Some diesel fuel injection services may clean it up, or it may need replacement if it is too bad.