I saw 19 mpg, 19.2 was my best, a few times traveling through Kansas and Oklahoma with a tail wind and cruise set at 70. That was a stock '07 4wd LTZ (3.73 gearing) with AFM still active and calculated using Fuelly.com. I was never able to get the computer's MPG numbers to match the actual calculated value. Sometimes it read high, sometimes it was low.
My Tahoe's "lifeime" average while using Fuelly was just shy of 15 mpg.
I would fully believe at a 2WD tahoe with 3.08 and the 6l80 to be able to beat 20mpg easily.
At the bottom of your stats page on Fuelly, it shows how much you would save by increasing your MPG. IIRC My Tahoe stayed around $4 saved with a 2 mpg increase. Right now for my Suburban, it's claiming $8.86 would be saved if I increase my MPG by 2 based off of my last fill up. The advantage if a bigger tank I suppose.
At $4 per tank at the rate I drove and assuming that everything stays the same, it would take about 18 months to "save" $400. That makes it difficult to justify spending much to improve MPGs. I could probably save more than that by drinking a little less beer or drinking R&R instead of Crown. Bleh.