Our ScanGauge display is often backlight unless we unplug it or until the next time the ECM is powered on and back off again.
It has done this many times over the years even when connected in previous motorhomes.
Had the same problem in my M2 chassis. Took it to FL and they said "no trouble found". However, after the visit the problem stopped...something must have been reset by FL. I would call FL support and ask them if there is some reset that you can do so the light turns off.
Ok, call off the dogs! Found the drivers side door switch wasn’t making. Grands had obviously been playing with the cab lights and they were in the off position. We never noticed cause we don’t drive at night. Simple things are so frustrating! Thanks all!