2002 Monaco Diesel heat and monitor panel

The heater in my rally compartment is 12 volt and is controlled by a temp sensor.
It turns on when the temp is in the mid 30's.

By the way, your post would be much easier to read if you would use proper punctuation.

Welcome to the Dark side...the DP side. Most of us over here are not grammar and pronunciation police. From a TC it's going to be quite a learning curve. Help us out by adding which Monaco and what kind of heating system it has as you may have a AquaHot system.

Inside on the main switch panel you may find some switch labeled 'System Heat' with a light next to it. When you turn on the switch it will glow red. If the temp is low enough outside then the power will go to the heater and the light next to the switch will glow. This tells you the heater is getting power.

There is a snap switch between the inside one and the heater that will not turn on unless it is cold enough and will turn off to stop the heater if the bay is warm enough.

In your manual look for 'System heat'. It is only 40 where we are and it never came on last night. In Michigan it would come on but only raise the temp in the bay about 20 degrees.

The system 12v heater in the wet bay is controlled by a "snap disc" this disc can be mounted anywhere in the wet bay..when the disc itself gets near 35 degrees it will snap the heater on and go off when it is 45 degrees..If you can find the snap disc, I'd spray a duster spray on it (it's cold) and it will turn on the bay heater for a test..These things have been known to fail and you don't want it to fail when you need it..After the test, it goes off within 5 minutes...My snap disc was mounted on the upper right of the wet bay behind some kind of panel..Make sure the system heat switch is on..and DO NOT DRY CAMP with this heater on...if you do and it goes on the batteries will drain very quickly..so if you feel it is getting cold and you need it..shore power or generator.

Look for the snap disc in your bay and follow the source....Dennis