I can understand your disappointment. Here, diesel cost is comparative to premium gas. And w/ the volume of oil and the cost of filters the trucks must be up to efficency or they are just not worth the expense. Though the torque of a diesel is very strong.
I have both. A 7.3 Ex and a 6.0. F350. An '03 and an '04 respectively. You can see my 6.0 milege up a paragraph or two. The 7.3 gets about 11 around town and 20 +/- on the highway doing 65 or 70. I pulled a car trailer about 1100 miles once in the 7.3 and got 18 HWY.
Does yours have the exhaust regenerator? (Or, whatever it is called.) I read, when they system first came out, that the MPGs were way down - 5 or 6 MPGs - due to the system dumping fuel into the regenerator to burn off the diesel fumes. You may want to check into that system if you have it. The system was one of the reasons I did not turn in my '04 6.0 at the time. By the way... I still have my 6.0, though I only have 80,000 miles on it, and it is running good.
Not having the regenerator system I have not followed the forum writings regarding it. Perhaps there is a way to improve, or bypass, it if it is the reason for the poor milege. Only legally of course. I am not suggesting you do anything illegal.
'03 Excursion, 7.3L, 4X4, Auto tranny of course (they all did), 4" MBRP exhaust, TS performance chip, K&N FIPK w/ an Amsoil EA filter, Hellwig anti-sway bar, drilled/slotted front & rear rotors. Rancho XL shocks, Rancho steering damper, Airlift 5000, S&S Diversified headlight mod. This baby surprises a lot of, so called, performance cars.
'04 F 350, extended cab, 6.0L, FX4, manual tranny, 4" MBRP exhaust, Bully Dog Triple Dog downloader w/ Outlook monitor (Very disappointed in the Bully Dog), 2m-70cm Yaesu ft-8500, S&S Diversified headlight mod.
'96 Mustang Cobra convertible, B&M short throw shifter, Koni adjustable shocks, viper chip, drilled/slotted rotors front and rear, rear sequential turn signals.
'11 Harley-Davidson Road King Classic 103 ci PowerPak w/ anti lock brakes & Smart Security system. It even has cruise control!!! Candy Dark Root Beer over Candy Light Root Beer