Could have been the light that alerts you to the fact the new DEF using exhaust system needed some attention.
If you've only used the coach under very light load conditions, the system can't clean itself very well. If this is the case, you just need to drive it under a more stressed load condition, or just for a longer period of time so the exhaust system can develop enough heat to do it's thing on self cleaning process.
Dan & Loretta, US Army Retired Aero Scout Pilot
2012 Fleetwood Providence 42P Class A/DP
Spartan Chassis, 8.9L Cummins 450HP