Yes to all the input

!
- Look SL reported Diagnostics (And if needed, get the ECM scanned at Cummins, as not all error codes to some SL's. Though yours in a 2009, should have a SL that will report most of them.)
- Check the Coolant Reservoir tank level. IF too low, as Dean pointed out, it can cause what you are seeing. And bad coolant sending units can happen too...
When you are running, how is power? What boost PSI are you seeing when working the engine accelerating or hill climbing? (If not seeing it go above 30 PSI from time to time, you may need to look for breakdown in the tubing from the Air Cleaner to the Turbo Inlet (many opportunities along this path, for a clamp or rubber elbow coupling to disconnect some.)
And your coach should have the CM871 ISX in it, which is DEF and EGR. A clogging EGR, and or dirty MAP sensors - can also cause an engine De-Rate.
Hope it's something easy to determine, and repair for you...
Best,
Smitty