I should elaborate a bit. Studies have shown that tractor-trailers with "full aerodynamic treatment" can reduce power demand by around 20-22% at 65 mph. At intercity speeds, the effect if negligible and it doesn't even become noticeable until you exceed 50 mph. A 20% reduction in the horsepower demand doe snot translate to a 20% reduction in fuel consumption, but Cummins has found it could reduce fuel consumption by around 13%.
For truck+trailer, "full aero" means stopping the flow from the sides in between truck and trailer as well as smoothing the flow over the top and against the front of the truck. The roof top wings you buy for pick-up trucks for $400 attempt to address only one piece of this, the flow over the top of the truck and into the front of the trailer. You have probably seen the huge hoods that some tractors have to smooth the flow up to the front of the trailer. You may also have seen the "wings" on the side of the cab that stop the flow of air around the sides and onto the front of the trailer. This is actually the larger effect on a tractor towing a big box trailer.
I have not found any similar studies on pick-up trucks and recreational trailers, so there is no real evidence. However, RV trailers vary much more in shape than the box trailers that commercial truckers tow, and they have many more sources of drag, e.g. roof skylights, fridge vents, awnings, satellite domes, etc. These are not affected much by the addition of a generic wing on the top of the truck cab. And nothing reduces the inflow from the sides around the back of the truck. Aerodynamic flow is a very complex thing and it is
my opinion that the addition of some generic wing device to the top of the cab has only a slight chance of altering the wind flow in a positive way and in sufficient amount to effect fuel economy. It is much easier and all but guaranteed that you can reduce fuel consumption by slowing down 3-5 mph. Cummins test show that every 1 mph above 55 mph costs 0.1 mpg. And it doesn't cost a dime...
If your husband wants to study up on this, he can read this article by Cummins called "The Physic of MPG"
http://www.everytime.cummins.com/assets/pdf/MPG_Secrets_Whitepaper.pdf