the better MPG market are the snake oil salesmen of the new millennium 99% of them don't work and if the did either oil companies would use it or states would mandate they be part of the fuel when sold.
those that do actually work are so expensive that yes your MPG does go up but your actual profit is either unaffected or drops because of the cost of the item.
to save fuel don't idle use the 5 min rule if your stopping for more than 5 min turn it off or if the door comes open turn it off.
Find the range you get the best mileage and stick with it,this will differ from load to load, heavier loads slower speed. if your truck is slightly under powered this speed will be closer to 58 if it's slightly over powered it will be closer to 64-65. it appears all trucks are different so know your truck and drive your load is the best course of action