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What is the make and style of your thermostat(s? Once I know that I can direct you further here. Or attach a photo of that to your post and I can tell that way. Having either a small digital multimeter or a 12 VDC test light would be helpful here if we need to do some testing. All follow ups are included in our fee.
Are the other 12 VDC appliances working? The fridge and water heater?
So the display is dark and shows nothing?
Not sure that went through.............the display on the thermostat is just dark and won't come on to program? Do you have the manual that came with it?
This thermostat is the only one controlling both A/Cs and the furnace?
I am attaching a link to the manual for this unit in case you don't have it. Do a reset on this to see if it will come on. IF so, then you can program it. Let me know what happens. See the last page for that instruction. http://www.primetimerv.com/download/ComponentManuals/Thermostat-DualZoneComfortControl-Dometic-OperatingInstructions.pdf
This may be a bad connection AT the ceiling control. The RJ-11 cord plugs into one of these A/C units. Try unplugging it and then back in again. It may have just gotten some moisture in the connection. You access that by removing the ceiling grill and it will be seen then.
Some of the ceiling boxes have a fuse. The system will operate on one battery, but two are better and if the DC incoming voltage drops below about 10.5 VDC it won't work. Are you plugged into shore power so the battery(s) are being charged? If you have a meter you can take a DC volts reading AT the batteries and see. When they are being charged you should see about 13.6 VDC there. If you see below 11 VDC that can be an issue, but then the fridge won't work either.
Take your meter and check as I outlined...........
Then that isn't the issue...........check the cord AT the ceiling control box and give me the part number off the box when you are looking at it so I know which one you have.
Yes..........and I will make a phone chat offer for you in case you feel that would be more helpful here.
If you can get me the part number off the control box or the model number of the A/C I can call Dometic Technical and ask them for some assistance on this. First time I have seen this model do this. They are open today if you can get right back to me.
Just checking back to see if you need more on this from me or did you get it resolved? Thanks!