Look in your manual if you have one for information about a "Battery Control Center".
Mine is located under the hood. Driver's side. Yours could be adjacent to the batteries. Or you may have to race out the small control wire.
When the chassis reaches a given level the circuit board activates the relay to charge the coach batteries. If the voltage drops it drops out the relay. Later models then ours, does not allow the converter to charge the coach battery from the converter.
You could have an issue with one of the batteries that drops the voltage down which causes relay to drop out.
How old are your batteries? You may want to have them load tested.