I can remember when my 6.2 used to get high twenty's for MPG. In the last year it has been going down now around 17-20. I had to advance the injection pump last year to make the truck run right, other wise it would blow alot of smoke at start up. I talked to a guy and he said that the pump is getting weak if I had to advance it to get it to run. It also seems to blow more black smoke than it used to, which would not bother me a bit if fuel wasn't $2.55 a gallon. It is a half ton, 2wd, with 700R-4 tranny(signature).

Poor injector spray pattern will cause incomplete combustion -> Black Smoke. How many miles? Could be time for a pump and injectors.

130,000 miles, Put in new injectors 10,000 miles ago. Pump was replaced by previous owner, but im not sure when.

How is your EGR function? How is your air cleaner filter? A slipping clutch torque converter would reduce your MPG. I would run a tank full with a can of diesel injector cleaner in it to clean off the injector tips. Injection pump housing pressure could be higher due to a restricted fuel return line. You could have a bad rebuild on your injectors. You stated they had been replaced 10K miles ago which is about a year of driving which was the start of your lower MPG.