We too just returned from a 2500 +/- mile trip down to DC, Outer Banks, Asheville (Rtes, 81, 95, 40, 26, 81 and state routes) grossing just south of 22,000lbs. Exceeded my expectations going up/downhills. Going uphill, she would if I pushed it, actually pick UP speed! Going downhill, just tapped the brakes a time or 2 and engine brake kicked in and she held speed to whatever I wanted it at. Hills of western NC and eastern TN were not only beautiful, but gave a good indication of how she would do fully loaded and 5 people. Smiles? You bet!
Used cruise when I could at 70 mph, A/C on and pretty much ave 10 mpg. While in Asheville, drove back to Charlotte w/o 5er and mpg's shot up to 18-19.
Didn't used the hill descent button as the eng brake worked as designed. Wanted to scale out when fully loaded, but raining pretty good at truck stop, so it was 3 days later when I did. Yes some food was gone, but not enough IMO to make too much of a difference.
CAT scale said truck weighed in at 8600 and trl was 13,200. Pin of just over 2,100 and truck weight w/ 5er of a couple hundred pounds south of 11,500. Overall, within limits of truck. Majority of truck weight or about 400-500 pounds of it was fuel tanks (Transferflow replacement 50 gallon tank). Compared to my old 6.0 (which wasn't bad), this is a beast of a truck and very happy.