*UPDATE*Calling Duramax owners 1st trip with camper bad fuel mileage

I had 07 cc lb lifted with 35s towing a 34 ft tt and a 40 ft 5ver.Never got that crappy mileage.I towed 65-75. I was getting 12 to 15 mpg
I did remove my cat first and helped alot.
Never threw a code.Then I went with 5 inch exhaust and did a egr black plate and did get efi.It made a big difference.
Then my mpg jumped up about 2 mpg and alot lower egts.
Now I have a lmm dually same set up and still get 15 to 17 mpg towing my 40 ft 5ver