Dometic Duo Therm Penguin 15000 btu kicks 20 amp a/c breaker

I would try and isolate the short you have in the 120V system. I would unplug the 120V wire at the control board and see if the breaker still trips. There is a fairly long delay before the compressor starts so it's not the problem. I would look for a black wire near the control board that has rubbed and shorted out.