ahh south Texan... still at it.
From someone, who has made it obvious hes not a diesel fan boy, this is potentially possible. Slow highway speed (relatively speaking), no wind, no hills, no hurry, lots of coasting, and factoring in that the dash is probably off by about ~10%, its not that unrealistic. Hes not getting that kind of mileage all the time, I don't think he was trying to say he was. Just that driving easy and trying to get MPG, worked. He was probably closer to 25 real world hand cal, but still that's legit.
while "trying" to get good mpg I have gotten (hand cal):
12 Mustang GT, Vortech T-Trim, manual - 27mpg - normal - ~20
09 HD 6.0L on 37s with 4.88s - 14mpg - normal - ~12
07 Powerstroke on 38s - 13.5 - normal - 9? lol
02 Camaro, 408cu LQ4, 88mm Turbo, manual - 23mpg - ~16
driving 50-55 and never accelerating and coasting all the time helps ALOT. But unless you are never in a hurry and got all the time in the world and dodge all the traffic and have perfect driving conditions, its just not realistic.
I should point out that ALL Of these numbers are based on FL driving (no hills and at about 20ft for elevation lol) AND done on a 30 mile trip at midnight with zero traffic.