So there's something to be said about owning a larger truck, fuel milelage is around 6 mpg, I may swap the hood over to the more aerodynamic one from the buss the one on the truck now is like a plough. With the swap possibly pick up another mile per gallon.
When I hauled campers for a living (if you could call it that) I added a scoop to the roof that I could take on and off, that way when I had a camper on I put the roof scoop on then as soon as it was delivered I removed it. It was worth about 1/2 mile to the gallon but when I ran almost 100,000 a year it added up to some real savings at the end of the year. At the end of the trip you could see the "bug splatter" pattern on the front of the camper to see how well it pushed the air over and around the front. This idea will work with any big load on the back or with a big inclosed trailer. It's worth 1/2 to 1 mpg. I bought mine but they are real simple to build.