Joe rough figures give about 4 gallons dripping all over everything in sight. Smell won't go away for months.
Keep in mind when some RV'ers throw around 19 it can mean a lot of things: They are lying, they weren't towing, they had a 40 MPH tailwind, they were driving 45 MPHnot towing, they were lying....
Our F450, only comes delivered with 6.7 and 4:30 gears, just like all those who tell the truth, when towing, 8.8 to 10 MPG. Real figures, although our lie-o-meter is very accurate. Not towing, 2000 RPM, 65 MPH, 16.8 to 17.5. Just about average unless I were to get it chipped in which case the HP would increase to 750, torque to 1500 lb/ft and mileage up to 35.
Okay, maybe the chipped part is campfire fodder.