These things would all make sense. I am curious if your mpg was always this low or if it changed recently. That could tell you if it is due to the winter temperatures or not.Things such as winter blend diesel, cold temps, drive line fluids being thicker when it's cold, snow on the roads equates more rolling resistance, warm up time is fuel being burnt without miles driven which equates to bad mileage, etc.
This is a trick I think really helped me the last time I was driving my truck. I found at 60km I could quickly lift my foot then apply pressure to the accelerator and it would allow to shift up, however I am speeding when I do it lol. however as soon as my wife starts driving the EVIC AVG fuel consumption climbs very quickly. In her words "I like it when I step on it and it pushes me into the seat!" She also drives my 2012 charger like that lolI see you are a jr. member as well. But something to be aware of that a lot of people are not on the forum is you need to reach your desired speed and 'lift' your foot off the accelerator for a second to force the truck into 7th or 8th depending on your speed. Unfortunately our trucks seem to like to hold the higher torque gears and this will drastically eat in to your mpg. Just a change of driving habit you have to make with this truck if you are not already.