If this is a Dometic CCC2 multi zone digital thermostat it gets its 12 volt power on the 4 Wire telephone cable plugged into the backside of thermostat. Remove thermostat and verify if Red and Black wires have 12 Vdc using a multi meter..
If power is present on the cable then thermostat is bad
If you have no 12 VDC on red and black leads of telephone cable you removed from thermostat then you have one of the following issues...
.. verify main coach batteries are charged and disconnect is not turned off
.. the CCC2 thermostat gets its 12 Vdc power from Zone 1 air conditioner control module mounted on the roof under its cover
... if main battery power is good then you can verify if you have 12 VDC present at electronic control module for Zone 1
Remove Zone 1 cover and locate control module and measure for 12 VDC power at connections Iíve circled in yellow..
If you have 12 volts at these two terminals on control module then either the RJ11 connection from control module to CCC2 thermostat needs reseating or you have a break in that comm cable running from Zone 1 control board down the thermostat
The location on the circuit board in the box below the A/C where the phone cable plugs in from the thermostat can also be a problem if it corroded. This was ours about six years ago, and a replacement solved the issue. Why it was corroded so bad was never discovered, but it is still working just fine. So, if no other obvious issues are located, give this a try.
Bob & Becky
2012 Chevy 2500HD D/A CC 4WD