Maxxforce 13 Fuel Anomaly

I have a a 2014 Prostar with the every so wonderful Maxxforce 13.
It's finally just to much, so I've decided to part with it.
One final conundrum just for curiosity,
I've noticed for a few months the fuel usage seemed top be higher than the gauge readout. I thought I was being paranoid until now.
I had to bobtail 600 miles. The readout says I was getting 12mpg - which is about right for a bobtail. But I went from full tanks (twin 100s) to less than a quarter full on the gauge. Sticked the tanks just to verify, and sure enough it used around 150 gallons in 600 miles, bobtail.
That would be crap mileage even fully loaded.
There is no sign of leakage. I have had frequent regen cycles lately and dense ashen smoke during regen.
I know there is a sensor for fuel usage, but that wouldn't explain the full issue.
Any thoughts?
Thanks

 

I've had this truck for a few years.
I was a lot more methodical about tracking when I first got it. The first year I owned the truck I averaged 7.8 miles per gallon. 70% of my Freight was above 40000 lb.
In October of last year I drove from Tampa Florida to Columbus Ohio with no fuel stops. 1100 miles roughly. And it still had more than a quarter tank.
So in terms of long-term tracking, I got used to my truck being so consistent that I really didn't pay much attention to it. But having such a drastic change in fuel economy in such a short time frame I was hoping that people would have The Usual Suspects list, so to speak.

These newer engines I'm just not up on. In a Detroit 60 if your fuel economy suddenly went to hell, apart from a leak, there are only a few things that could cause it and most likely you would be seeing smoke like coal, or diesel in your oil. With this maxxforce you see practically nothing out of the ordinary except for the fuel usage.