freightliner century poor fuel mileage

I have a 2007 freightliner century with a Detroit 14L 60 series.
I have been getting poor fuel mileage since I got the truck about 7 months ago. When I first got the truck it was about 4.3 loaded and about 5.6 empty pulling a tanker. I have already change my turbo, EGR value, delta P sensor and had it pressure tested for leaks after changing all those things it now getting about 5.8 loaded and 6.8 empty and its blowing back a little black smoke out the stack. Like when I go to pass a person and I'm on the gas it will smoke the whole time in on the gas and when I going down the hwy on cruise and I have my jakes on and when I it the brakes and they come on it will blow out black smoke and the last thing is when I'm pulling a long hill my check engine like will come on and soon as I let off the gas it will go off or stop pulling the hill I'min need of a little help with this have anyone had this same problem

 

Do u know I freightliner can do a leak test

 

Ok think I think I'll try the Detroit shop

 

No I keep the fuel filters clean I change them every 15000 miles