Best MPG will happen at the SLOWEST you can go while staying in 6th gear. As soon as it drops to 5th, your mileage goes down.
The mode button set to ECON allows you to get into 6th gear at a lower speed than when not set to ECON.
I keep mine on ECON all the time, keep my speed at around 58-62 MPH and consistently get 11+ miles per gallon. On complete flatlands (like going across Kansas, for example) and not pushing a headwind, I've gotten right at 13.
In fact, when I picked up the coach, the ECON mode would not work. FL replaced the shifter box and that's when I noticed my MPG increase by around 10%.