You can believe what you want, but you should read up on the bulldog system. I installed that in our truck shortly after purchasing it. You can use the device to reprogram aspects of your computer. It's a great little system. When we first put it in our truck started off with around 8mpg and slowly it has been climbing up. I think we are at the height of it now.
No I do not take my rig into the mountains. We go to Lake Ontario in September for a fishing trips with the guys. That trip knocks my mileage down a knotch or two. The nice part about the added tank is not having to stop and refuel means no getting back up to speed so I can maintain cruising speeds longer. Hence keeping my mileage up. i also do not go about 65 with my vehicle which keeps me in the green for mileage.
The point I was trying to make by posting is if you equip your vehicle the right way you can do what you want. I have added many different options to the truck to help with mileage and they seem to be paying off. Also my truck only pulls my trailer it is not a daily driver.
Sorry to the original poster for causing such a disruption in your topic.