From what I've read the ECO mode does not really have anything to do with MDS.
Seems like a lot of people think that because "ECO" is displayed then the trucks MDS is active, or whatever.
From what I understand it's just there to show you that you're "driving in a fuel economically correct way". You'll notice if it's in ECO mode and you give it just a little bit too much gas, it will drop it, let off a little and it will show again.
Basically as long as you aren't accelerating too fast it will show ECO. Or if your foot is off the gas altogether.
I could be wrong, but that's what I gathered from reading the manual and looking around online