All of the OEMs spend a fortune on aerodynamics. We have variable speed water pumps. Our transmissions have figured out how to safely let gravity and inertia move our trucks. These transmissions know if you're heading uphill or downhill and will choose the most efficient gear. Even the mirrors are aerodynamic. All of this can't stop the biggest MPG thief.
The biggest MPG thief is at work when the truck is standing still. Idling is the biggest MPG thief. My idle time percentage is 6.05. I could definitely do better. I tend to leave it running as a quickly inspect the trailer that I am dropping to make sure that the lights work for the next driver. I drop the trailer and then hook up to the next trailer, shut the truck off and check the rest with the truck turned off.
I see a lot of engine reports. Engine reports can be 46 pages long. Henry loves to dig into them. They hold a lot of valuable information. There is information deep into the reports that a driver or shop can learn from. My eyes are immediately drawn to the idle percentage. It is the low hanging fruit of the engine report. I see reports with similarly equipped trucks in like operations, one may have a 15% idle time and the next one has a 45% idle time. Guess one get the better MPG. Reduce your idle percentage and your fuel mileage will increase. As much rocket science that is put into modern trucks, reducing your idle time is just a simple decision.
I definitely recommend some sort comfort system for your truck. My park smart system is a factory 
battery powered APU system. It has definitely spoiled me. It hasn't always been this way for me. My first anti idling device was a good sleeping bag. Most of my career has been in the upper midwest. Battling cold was more important than battling heat. I would run the heat in the bunk full blast before I shut the truck down for the night. Then I would crawl into my sleeping bag. When I bought my first new truck, I got a bunk heater and I thought that it was the greatest thing ever. In the summer time, I would blast the AC for the last hour of the night. Usually the truck would stay cool enough to fall asleep.
Thankfully these days we have many options to reduce or eliminate engine idling while the driver sleeps comfortably. If you are purchasing a truck, do your research. Get the idle reduction equipment that best fits your operation. Don't forget old school. Close the bunk curtains. On hot days, try to park the cab in the shade. On colder days face it towards the sun. When it is cold park between trucks that are running. I still see trucks idling all night in good sleeping weather. Do everyone a favor, and shut it off.