For anyone who may have this issue in the future, I finally figured it out. I replaced my interior climate control unit since the buttons stuck. I believe the replacement I got must have come out of a 2014 Silverado 2500. The 2014 Duramax's had a coolant level sensor, the 07-13 1500's do not. That sensor must run through the Climate Control unit because after plugging the old one back in, the message went away. My local Chevy dealer could not figure it out either, however, because that particular message is not one that is listed for the 2013 Silverado 1500. So long story short, a new climate control module out of a 2012 1500 solved my problem. It was never an actual issue with the coolant system, just the lack of a sensor causing the message to appear.