I've logged 20.5 mpg in a '05 4x4 Expedition with the 5.4L V8. This was along I-17 from Phoenix to Flagstaff, so there was some city driving mixed into that average, along with considerable change in altitude. Went from 1500 ft to over 6000 ft elevation on that leg.
With the right driving habits, decent mileage is possible with a V8 equipped vehicle.
How did I manage that? I kept the vehicle pegged on 70 mph on the flatter sections, and on the long ascent, I turned off the A/C and took the climb at a steady 45 mph on the steep sections. I was keeping up with the normal flow of traffic on the hill climbs, too.
You'd be amazed how much improvement you'll see if you keep it under 70mph.