There is simply no way you can run 2 fully functioning RV AC units from a 20 amp source. Go to any conversion equation web site and convert the BTU's of you AC units to watts. I'm guessing one of two things. You aren't hearing the compressor, you're hearing the fan or secondly your AC unit is bad and although the compressor is running, it's not working hard and is drawing less current.
Again, the laws of physics say you can't run 2 13, 500 (or 15,000) BTU AC units on 120 VAC and 20 amps. Energy is energy. Even if the compressor were 100% efficient isn't going to happen.
And quit knocking the folks that are just trying to help.
