My best towing is 16.7mpg towing my dakota on a 7k trailer @ 70mph with 3.55s and 33" ish tires.
2001' 24v Dually 47re trans
100hp injectors, AFE intake, 4" exhaust, HX35 with a boost elbow, Edge EZ chip
Empty I can hit the 20s (these #s are hand calculated btw)