Yup. There's *two major things. Vehicle/glider efficiency compared to speed, and engine efficiency compared to speed. Since, in just about every instance I've seen, the power required to move a vehicle in OD drops faster than engine efficiency drops in OD, going slower will almost always result in better mileage, at least at speeds above ~30-40mph. While going slower and getting better mileage means that overall vehicle efficiency is increasing, it doesn't mean that drive train efficiency is increasing. It's actually decreasing. Just not as fast as the vehicle efficiency is increasing. So, you see a net gain in mileage.

For instance, assuming your truck has a Crr=.015, Cd=.45, A=4m^2, and W=20000N, at ~25mph you'll need ~6hp, and at ~100mph needs ~168hp. So, you need 28 times the energy to go 4 times farther in an hour, which means your truck needs 7 times more energy per mile to go 100mph compared to 25mph. Otoh, I doubt by going from 100mph to 25mph you'll see a seven fold increase in fuel efficiency because at 100mph your engine is operating at peak efficiency, while at 25mph it's operating at much less than this, but the differences are still likely a factor of 3-4 compared to a factor of 7 for power.

*Well, actually there's transmission, driveshaft, etc.. But they aren't as big.