Fuel economy is a physics formula... . based on wind resistance, rolling resistance, acceleration, and weight..... There are very few things that will change mpg except the items above. . Dodge would jump at an improvement that would give them 2 mpg based on C. A. F. E. , (corporate average fuel economy). CAFE is a federal standard based on the sold vehicles and their fleet... so for every big truck sold they need to offset the sale with a small vehicle or two...
The key is to reduce weight, or any of the fixed items listed above... its very hard... the only thing that I have found that functions for me after several hundred hours using a dyno years ago is to accelerate slower and drive slower to reduce the wind resistance...
My wife driving the same route, vehicle, will easily get 2-4 mpg better economy because she accelerates slower and will drive 7-10 mph slower than I do... .
We used to offer any company selling their products (snake oil) dyno time if the product improved mpg... we had the ability to set the vehicle up and measure fuel flow under load... nothing we ever tried ever improved mpg except slick 50 in the transmission and rear end... there was an almost an instant improvement BUT we found that the product had to be re-introduced every 15-20K miles to be effective... and wasn't cost effective... .
My personal thoughts is there is nothing on the market that will do what your asking for that will give you a suitable return on your investment over say 25K miles... . its easy to make it go faster, accelerate faster, run smoother etc... .
I own several trucks, my fuel use is close to 1500 gal a month... I'd step up in a heart beat for 2-3 mpg... with a payoff in 10,000 miles. . Trust me on this...