I have a 2016 Pro-4, so no roof rack but it came with the Hankook P rated A/T tires. Honestly everyone has it about right here, it's what kind of driving you do and how you use that right foot. You can do a lot more I believe with the manual, but I've not seen anyone with an automatic that I could try so who knows. There are days I am just aggravated or whatever and subconsciously the right foot just hits that pedal a bit harder, other days it's not nearly as bad. For me, a lot has to do with what type of SXM channel I have on, whether it's calm and soothing music, or other.
That said, the best way I feel to determine how your vehicle is doing is to avoid all of that, and only look at what it's doing when you're on cruise control. Generally doing 60 would make the most sense. That removes the whole "driver" aspect of the equation at least. I have tossed on some pretty heavy addons in this first year, shrock rear bumper + full skids as well as some rather heavy duty rock sliders, and I have noticed it's not really capable of maintaining up to 20 mpg anymore even on generally flat highways cruising at 50-60. Not unless I go to premium gas at least. The truck is only going to get heavier as time goes by and I'm allowed to further abuse my bank account.
All that nonsense said, I generally try to aim for 18 mpg at the end of a full week and fuel tank for my DD to work and back which tends to be 2/3 highway driving and 1/3 stop and go city. Sitting at 18.3 by the stock truck MPG meter as I parked it this morning, and I reset the meter each time I fill the tank up. Worst I've ever seen was around 9.5 mpg when towing a dive boat that was at or exceeded our towing rating, best I've seen was about 21-22 mpg with cruise on and full premium gas.