On our discovery the single thermostat can handle up to 3 units. The 3 heaters/air conditioners are assigned a number address via jumpers on the Load Shedding" circuit board located near the Transfer Relays and main power circuit breaker panel. In our case only 2 jumpers are used thus only two units are controlled.
I had a problem similar to you. The thermostat got confused and thought there was 3 units and did not find them...I pulled off the cover of the thermostat, pulled the power cable off the thermostat circuit board, waited a few seconds then plugged in the connector. This fixed my problem.
If not already you should contact REV and get all the drawings they have on you coach. Provide you serial number make, model.
REV contact info: firstname.lastname@example.org