I did a little bit of playing with a spoiler but did not see an increase in the MPG. My testing was on some short hauls (25 miles) with and without the spoiler and same roads same day. I did see some improvement the one time I was towing with my trailblazer and had kayaks on the roof in a vshape. I plan to do some testing more with our current tow truck (Dodge Ram 1500) and play with the placement of the kayaks to see what works or not.
One thing you need to be careful with is how much you spend to get the increase in mpg as if you only tow a few hundred miles a year you will not see a return on investment.
On thing I have notice an increase in is Full size tractor trailers that have an apron under the trailer which they say increases the MPG of the tractor trailers.
Some of the testing or Work I plan to do with my Dodge Ram 1500 to increase my MPG is Cold Air Intake, Change the muffler system out, computer chip, Air deflectors around the windows and hood. Any saved MPG on that beast will be money saved in the long run as this is also the truck I use daily when the weather is bad and I have to work. Nice days (no rain, snow or ICE) I run a motorcycle including 30 degree days to work (75 miles round trip).