won't work to save fuel, will be worse
The ones mount on your cab.
With a trailer, your front (face)of trailer is about 8-10 feet from cab, now it will throw air up and form a turbulance of air coming back down into box and then try to again go up the front of trailer.
your best bet is to have a topper on your truck, this will work much better as it channel the wind back at same hieght as truck cab, and then you're only around 3 feet to face of trailer, with the slope of front end it's good air flow.
I have a canopy on my truck. It is also nice to have covered storage for camping stuff.
I had a deflector on my Astro van when pulling my old prowler, flat front end and I mounted it at the very back of van body to throw air up there. it helped but I was only 3 feet from face.
same as the 5ers they face is 2-3 feet from cab