I have a 2004 GMC 4500 towing a 21,000 lbs. trailer and going up a long hill in Virginia and the turbo just got quite and stopped working, lost power, lost fuel mileage and lots of black smoke. What should I check first.

 

Check the connector on the VGT solenoid... Push it on and off a few times. I'm no D-max expert, but it's worth a shot...

 

check air cleaner, make sure not clogged. happened to me once on my work truck. plastic bag got sucked in.

 

Was there a boom? You may have blown a boot off one of the boost tubes.