I don't have a hard number. This one has a single 73 gallon tank, and the recommend only putting in 70 gallons due to expansion. Each time you hit a fuel stop the pump stops at either $75 or $100, so I didn't fully top up until I hit an actual high volume truck stop pump. $148 later... Then there is no real obvious point to stop pumping fuel. I made an estimate from the fill up and pumped to that height on the subsequent fill up. My calculations from that came out to 9.83 MPG. I am now using a higher line as there was a lot more headspace. Part of the learning process when you are unfamiliar with these trucks. Next fill-up should give better results, as the line I chose is more definitive.
Also when you pull all of those hills out of Phoenix to Flagstaff in 110 degree heat, it really does not help the mileage. After getting to the top I set the cruise at 61, put in ear plugs, and settled in for the trip. One of the first mods will be to add a lot of 80 mil sound deadening material to every square inch of the cab. There is literally nothing between you and the noise except a bit of plastic, a rubber mat, and sheet metal. Then add dampener on top of that, and re-assemble. The noise is mostly the air intake when you are in the throttle on big climbs. Higher pitch, and wears on you without earplugs. Cruising is not bad though, even without them. Could listed to the radio and all that.
Also think the air filter needs to be replaced, or blown out. That should help with the intake sound a bit. I noticed the intake gauge in the dash was into the yellow a bit. It was fine, until you had to do a big climb, then would go up to about 8. Spent a few years with Humvees in the Army, and never saw one of those gauges so much as move a tick. Of course this thing pulls a LOT more air, so it may be normal. Worth checking either way, and could have changed the mileage a bit as well.
Still learning.