These are called air tabs. From Tritons web site they say these break up the airflow on the back of the trailer and actually increase your mileage. Kind of interesting, but it might be one of those things that are hard to actually test and quantify, unless you are running lots and lots of miles.
Wonder if the trucking industry has tried something like this. Seems like they are on the cutting edge since a semi really racks up the miles and even a small gain can save a lot on fuel.
Some European cars have had those built into the body work for years. I remember seeing them on Citreon cars quite a while back but I think it cut down on aesthetics so they didn't really catch on here. Could work on the trailer but might be a bugger to keep the road debris off.
I have seen these in the trucking industry for years. They are similar to a vortex generator that has been used in the aviation industry. I think there would be a small gain in fuel mileage but not a huge one
Nowadays alot of semi trailers run fiberglass skirts that run from the landing gear to the axles, and wings on the back doors on both sides and top and bottom that sitck out behind the doors about 3 feet and angle in slightly. Look cheap as crap but some drivers claimed they helped a bit.