I th iknk the qquestion is just how much does it improve mileage if indeed it does. I bought one for simplicity and use it mainly for a visual clue as to how my truck is positioned prior to hooking up. Although I havew a bed mounted 12V connector for my 7 pin,,,the cap broke off and so I use the stock one under the bumper...the V tailgate makes it easier to grab the cable and hook in.
Where I really notice a change in mileage is the fact that the nose of the 5th is significantly closer to the cab and is sculpted...that fact has given me on average about 1.5-2mpg better than pulling a bumper tow of any weight.
Stk
I really could care less aobut the fdifference if in fact there is on. Without the trailer hooked up, I have a bed cover.