2017 Chevy 2500 4x4 Diesel running 75-80mph is around 16mpg, truck only has 10K miles so I would expect 1-1.5mpg gain over time.
If you don't go over 65mph it will do a solid 20. With the fuel shortage I started pinning it on 65mph. Was really surprised.
My 2015 with 120K miles and 35" tires would do around 17 running 75-80mph down the highway.
Towing is mixed depending on how heavy. My boat with get 15-16 mpgs doing 75. Tractors or stuff like that around the #10K mark its more like 11 or 12. Start pushing that #20K mark and it will go single digits.
All my trucks have oversized, heavy mud terrain 10 ply tires, several hundred pounds of tools and feed, bumpers, etc.