Head winds, tail winds, speed, stop and go, highway versus side roads, and finally mountains. Everything changes the mileage.
I kept track of every gallon of fuel for 70K miles in my 2010 Ram and am also doing this for my 2018 Ram. The trucks are similar, CTD with crew cab. The camper is identical a Northstar Igloo. Final weight for both rigs was about the same at close to 13K pounds.
The 2010 averaged about 12 mpg and rarely exceeded 13 or so on the highway. The 2018 does noticeably better at about 13.5 overall and often 15 on the highway.