A lower to the ground front deflector on the tow vehicle will do the most good.
Newer Rams have the front deflector set up to lower 3 inches at speed.
TFL has tested trucks with a tonneau and without for good provable improvements.
I can see where the deflector that far back should do some good but hard to measure/prove. Side winds can play havoc with MPGs.
On our previous dually/fiver Cummins running at 65 rather than 70 we picked up 9/10s of a gallon.
I haven't played with the new van/TT for mpg as we have only done 150 mile or so trips so far.
