Head scratcher... Maybe a bad connection somewhere that is intermittent? Take a voltmeter and check the voltage to ground from each 120 volt breaker in the panel and make sure each circuit is working at the panel. I found one totally loose wire at a breaker in our KZ TT. Check ALL connections at the converter/panel for tightness. Make sure incoming 120 volt 30 amp connections at the panel are good. 120 volt issues like microwave only working should not be affected by the 12 volt system like that.
Maybe whoever added the 2nd AC caused something?
BTW, you may find it a problem to run two AC units at once on 30 amp, especially if the voltage gets too low. What is the voltage at the panel in the trailer when connected to the house? Low voltage in CGs is a common problem.