2011 Mercedes-Benz M-Class - Stalls

My ML 350 2011 BLUETEC stalls in the morning after being left overnight. once i put it in reverse it shakes and stalls. Also my gas consumption has drop by almost half. when i got the truck i was getting 980 to 1020 km per tank on diesel, now im getting 560 to 620 km per tank.

Is the check engine light on? If so, what are the codes?