Its not that hard. Under certain circumstances only a fraction of the engines power is required to keep the vehicle in constant motion. The bread and butter of the AFM is on the highway going 50-65mph, which is the average speed limit of US highways. At those speeds is where the truck is most aerodynamic and when the v4 can keep the truck at a consistent speed.
It is really hard work to get a 6000lb truck to get over 20mpg. That is why things like the air dam, tires, and ride stance or so important and sensitive at the same time. Just take a look over at the suspension/leveling thread. When guys level their trucks and remove the air dam and add larger tires they can take a hit of up to 4mpg.