Bryan.....I owned a 2005 Diplomat for nine years and did all of my own servicing. For the life of me, I can't think of ever seeing an ATC control box. I do know that on one occasion when the light stayed on, it was because the sensors at the wheels were covered in mud. There are sensors on both rear wheels. Make sure they're not broken or have something stuck to them. (dirt or debris).
The light will often come on after the coach has sat for awhile or it's been rainy and wet. In those conditions, I would drive the coach for a few miles. If it didn't go off by then, I would stop and restart the coach and the light would go off.
There is also an ATC switch....make sure the switch is good and not stuck on.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab