I pretty much agree with turtle n Peeps. The caviet is that things that could improve mileage would be a smaller engine displacement with less HP. less swept volume= less frictional losses, less heat loss which helps mileage.
Also, lower boost and higher static compression will help thermodynamics, the higher the compression at no boost, the more efficient. Notice that the current diesels are down near 16:1 compression, likely partly to support the higher boost. higher compression, less boost at light load would help mileage but hurt HP.
All in All, these would likely help unloaded mileage way more than towing mileage. Towing the large frontal area and very high Cd pretty much dominate everything and swamp out most other improvements.
BTW my 2004.5 LLY duramax would get at best 17mpg average on a trip between Portland Or and central montana. Our 2015.5 LML same configuration but 600lbs heavier, 7600lbs vs 7000lbs with the 6 speed gets high 19's and low 20's on the same trip, even though the speed limit in Idaho went up 5 mph (75) and 5 mph (80mph) in Montana. So the extra gear along with better effiency in the motor help the unloaded mileage.
Towing between the two yields virtually identical mileage.
I think one of the best ways to improve mileage would be to add start stop technology to pickups along with a mild hybrid system. Battery bank big enough to absorb the kenetic energy from say a 50mph stop, with maybe an option for a big enough bank to absorb kenetic energy for a 20,000lb combined weight down a 4 percent, 1 mile grade.
the first would be a big help on unloaded in town economy, the second a help when towing over up and down hills.
