Severe drop in gas mileage LM7

Hey everyone, I have an 03 1500 Z71 5.3/LM7 with 3.73 gears in it. Up until about a year ago, the truck could get 14mpg in town and 17 on the highway. Even after the leveling kit and tires it would still get that. One fill up, I have been getting 12 City/ 13 highway in it and nothing has changed that should cause it. I have already changed the plugs in it. It has 153k miles on it and runs great. I thought maybe the fuel filter was clogged but the truck seems to be running rich because it will haze black smoke under WOT. Anyone experienced this and or know what's wrong?