About the same here, towing the 17Z with our Silverado our best was 13mpg, with the wind behind us, worst was 8.5mpg, strong headwind. Typically 10-10.5mpg. We are getting about the same results towing our larger, heavier Starcraft.
Not sure why both vehicles seem to struggle, both of those engines should have plenty of power to pull an X17Z.
2013 Starcraft Travel Star 229TB
2011 Jayco X17Z(traded in)
2010 Jayco Jay Series 1207(traded for the 17Z)
2011 Chevrolet Silverado 2500HD Z71 CC 6.0L/3.73
Reese "Pro Series" 800lb Trunnion WDH w/sway control