Not enough information given to hazard a guess.
T Stat brand with error code would be helpful.
Inverter brand and model also would be helpful with error code.
I guessed on the T stat to be a Dometic and E1 for that brand per
HERE is..
"The E1 code indicates lack of communication between the CCC 2 thermostat and the power module boards. The system cannot operate and is about to shut down. Also, it may signal lack of communication between the CCC 2 thermostat and an individual power module board. The code contains the number of zone, which is out of communication, and it will appear on the LED screen. If an additional zone loses contact, it will blink next to the current one."The A/C unit has a control board that digitally talks to the T stat via a cord that looks like a old style phone cord (but it isn't it is wired differently). In a nutshell communications between T stat and A/C is not happening any more.
Also instead of checking voltages at random places, start right at the beginning of your 120V system chain.
Check voltage at the RV end of the shore cord as well as at the generator.
Check voltage at your EMS input as well as the EMS output.
Check the voltage at the breaker panel on your main breaker.
Check the voltage at the inputs of the ATS and the output of the ATS.
Write everything down and then compare if or how much voltage drop you see between inputs and outputs. Where you find the greatest voltage drop under load you have found the problem.
Check the battery voltage at the battery and at the Inverter (you really need to give the brand and model or search the Internet using the name and trouble codes). Those error codes may be output overload, charging failure (if equipped with charger), too low of battery voltage (charging failure of the converter, faulty or worn out batteries).
