Open (turn off) every breaker on the AC panel. Pull the plug
on every AC connected device. Turn on the AC (either Gen or shore). Sounds like at this point you get "AC Available" light? Close one breaker. EFCI trip? If not, try to run whatever is connected to that breaker. If it doesn't trip re-open breaker and wash/rinse/repeat with each breaker one at a time.
If you've already gone that far and have trips with more than one load then it has to be in the common neutral wiring
. EFCI is expecting the same current
in the hot and neutral legs. If it doesn't see it then some current
must be going elsewhere. Working from the point where the shore power and gen power cables
meet, check the neutral and all connections from there to the AC panel. Likely one is somehow broken/damaged.