You need a code scanner/reader to know what is going on. I recommend the ScanGauge D. https://www.scangauge.com/products/scangauge-d/
It's inexpensive and in addition to reading fault codes, it can also display engine and transmission data (coolant temp, boost, oil press, rpm, speed, etc.). There are dozens of engine and trans data elements it can display (four items on the screen at a time).
Or take the Coach to a shop and have the codes read. Don't take a chance hoping the light will go away by itself.
PS. There may be a way to read the fault codes without any tools/readers. Many diesel Coaches will let you read fault codes by counting the number of check engine light blinks. In those Coaches you activate the "code blink" using the Cruise Control switches. Check you Owners manual for the instructions.