2005 Chevrolet Silverado C/2500 6.6 L V8

I would like to buy 2005 Chevrolet Silverado C/2500 6.6 L6.6 L V8 diesel truck. Do you guys know what would be MPG for this truck. I own 26' 5th wheel (DW 7,900 GW 9,900). Thank You.