12L/100KM is not unheard of, but I wouldn't expect it all the time, especially if you live in a windy area. I managed to squeak out 11.9 my last highway trip (19.8 MPG), and that was with my 3.92 gears. The catch... I was limited by Minnesota's lower speed limits, with long periods of highway construction which slowed me down even more. That helped a lot with the gas.
Here in South Dakota, with the speed limit at 80 MPH (just under 130 KPH) on the interstate, if the wind isn't terrible, I usually average closer to 14L/100KM. If I fight a wind at all, which happens at least a third of the time, I'm closer to 15-16L/100KM. Again, that is with my 3.92 gears, which will never get quite as good mileage/km compared to 3.21.
If that truck has been test driven frequently, it will show quite high fuel usage. When you test drive, people will take it on short drives, accelerate quickly, get a feel for how sporty/powerful it is. That equates to lower gas mileage (higher L/100KM).