The OP was looking for first hand experience and has gotten one response fitting that request. You have a sample size of "one" to make your decision. I, like many others, used to see a few of these years ago but see very few these days. That alone should give you some pause. Getting a 2 mpg boost would really be something.
I knew a guy years ago who read about a new "top secret" way to increase your mileage in any internal combustion engine. The story went that the government (yeah) knew all about this but wanted to keep it from the public for fear that it would seriously hurt the petroleum industry. The secret was to tape two skinny magnets to opposite sides of the gas intake line of the carb. He did this, loaded his family into a huge 70's vintage station wagon and drove to California. Came back and swore he got "better" mileage. Never said how much better. Point here is that when people buy in, either monetarily or emotionally, their statements are often biased. Now, lets talk hitches and tow vehicles, shall we?