The costs for truckers work out because they drive 200-250K per year.
The under skirts on trailers give 0.3 to 0.4 mpg improvement under 65 mph, about 0.1 to 0.2 mpg at 75 from what I read. If you put eight thousand dollars into a trailer, you might get 0.7 mpg improvement.
If you run 250,000 miles per year at 8.1 mpg vice 7.4 - you burn 2,919 fewer gallons of fuel @ $4 per gallon - you save $11,676 dollars per year. A trucker can also save about 0.25 to 0.3 mpg by going to wide fat rear tires rather than duals - at about $1,200 per axle.
If you tow 25,000 miles per year - a 0.7 mpg improvement is a savings near $1,676 per year.
Dropping your cruise speed from 70 to 60 will save about the same amount, to 55 another 0.6 to 0.7 mpg.