My Avalanche has the 5.3L. I get 15 MPG mixed, 18MPG freeway only. But the Avalanche is heavier than a Silverado (sits on a Suburban frame).
Motor wise, I had to replace the alternator due to the bearings starting to make noise. Otherwise, nothing. Tranny has been perfect; I did add a good external cooler when I got it. Up to 118k on the truck now; owned it since 68k. Had a cat converter's internals break loose @ 116k...I, ummm, "cleared out" that issue.