When you turn on cargo heat inside the RV the heater does not go on immediately. There is a thermostat that senses temp. It only turns on when it gets down to about 35 degrees. My coach, and I suspect your coach also, has heat pads on the grey and black tank. The thermostat is supposed to also turn them on with that same switch. I have not looked for that thermostat. I don't know if that thermostat is in the heater or just somewhere else in the wet bay. I would call Monaco and ask where that thermostat is located. 
That does bring up another issue. Is it your heater or is it the thermostat?