Calling SE can answer a lot of your questions.
Pleased with the install they did for our coach, which looks like Don's. When the had the radiator return pipe off to braze/weld the attachment into it, I went and bought a new thermostat. Figure the OEM one was 14 years old, so while it was apart, might as well replace with a new one.
I have not noticed any change in MPG, I would not be surprised to see Don's next run may show the same, or perhaps a bit better (Sure not enough to write home about
!) then OEM Fan Controller. The wax valve is infinitely variable based upon temperatures. So the fan RPM does not only have Low/Medium/High fan speeds to jump too. My bet is the bulk of the time, the fan will be running at lower RPM's then the OEM Fan Controller would have it running at.
Best of luck to you,