It is the bedroom tempeture sensor.
Found this in the manual:
Zone Control Zones are established at the time of the installation of your Dometic CCC 2 thermostat. A zone is an area of cooling/heating which is controlled independently by the CCC 2 thermostat. The CCC 2 thermostat allows for four zones (Air Conditioner/ Heat Pump) to be set up and run independent of each other. If you have one air conditioner/ heat pump installed, you will have one zone. If your RV has more than one cooling/heating system, you may have 2, 3, or 4 zones.