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Make and model of the A/C and brand and style of your thermostat?
Do you have a model number and the part number off the thermostat? It is usually on the back. I need that information in order to direct you in some testing. Coleman has dozens of variations in both.
lt is usually on the back and if you take it off the wall, you can see it. OR tell me how many wires are coming out of it.
You will need a small digital multimeter for some testing here. That thermostat should have a fuse inside if you remove the cover. There will also be a fuse in your load center which may be identified as a fuse for the furnace. Some RVs are wired up so that the furnace circuit supplies the 12 VDC to the A/C controls.
The red should be the incoming 12 VDC positive lead and the blue the incoming 12 VDC negative lead. You should be about 12+ VDC between the two. A meter can show this. Is this a digital model or analog slide control?
Is this a digital or analog thermostat?
No part number on the back? 4 numbers a letter and then 3 more numbers?
Go to the attached link and print this out. It gives the wiring and colors. With your meter you need to verify that there is 120 VAC to the A/C up in the ceiling. This can be an issue with the 12 VDC side or the 120 VAC side. A meter can help narrow this down. http://www.cleggind.com/postal_tech/pdfs/AC2/airxcel_1976376%5B1%5D.pdf
Just checking back in to see if you were able to resolve this or do you still need some additional input from me here?