With the mileages you quoted ,I dont think I would be happy at all.One thing you have to consider is that machine is NOT BROKE IN. Takes about 60 k miles to break in most diesels.
Also keep in mind its not a gas job and doesn't respond like one. Be gentle on that throttle partner,push down slowly and watch the fuel mileage go UP.
I get with the 7.3 about 14 city and 20 highway NOT TOWING and about 15 to 17 towing depending on conditions and traffic.
As soon as I fill up I will let you know what kind of mileage I got from Jackson/Meijers to Home on 127 from last weekend.
Roger & MaryLou
F350 CREWCAB SW LONG BED
7.3 liter Power Stroke Diesel
1977 27ft OVERLANDER
AIR # 22336 TAC- OH-7
May your roads be straight and smooth and may you always have a tailwind!