The all-time best mileage I got out of my '99 GMC 'burben was 20 (highway). I do a lot of driving up and down the I-5 corridor between Portland, OR and Tacoma, WA (135 miles approx. one way) so that's always been a great way for me to consistently monitor my mpg's.
This, of course, is the result of a few aftermarket parts like a K&N cold air box, MSD box/wires, SLP 160 deg. thermostat, Headman headers / Magnaflow cat-back exhaust, etc. I think the HARDEST part about achieving a consistent 20 mpg was trying to stay below 65mph to keep the aerodynamic drag to a minimum. Apart from those few lucky times of hitting 20 mpg, a majority of my mpg's have been more realistically around 18 mpg... which I am always more than happy with.
To be honest, for as big and powerful Suburban's are, I think they get pretty decent gas mileage when compared to most other trucks out there on the roads.