I average somewhere between 13-14mpg towing 4-5klb boat, depending on load. When towing, I'm usually +/- 5mph over speed limit. I too consume a lot of DEF. On an 1,100 mile round trip, I left with full DEF reading, added a jug once there and another once back.
Purchased a Z71 RST in January. The day after purchase, I traded in tires and bought BFG KO2 A/T. I figured I would share that 275/65r20 (~34" tire) fits perfectly with no modifications to suspension/level kit. Truly love the truck and MPG is incredible. My only complaints are the amount of DEF...