Fact: When my coach was new,, Sometimes the front A/C would work but not the rear, sometimes the rear but not the front, sometimes both, sometimes neither.. I was.... HOT, and not just under the collar.
Finally rather than return to the dealer I decided to trouble shoot myself, After all. I DO have the training, and a bit of experience.
First stop was the breaker box.. I went to the power source and took my volt meter with me, Never used it.. Noticed one of the wires to one of the circuit breakers seemed to be not all that well clamped.
In fact, It took over 2 FULL TURNS of the screw before the clamp even made contact,, (over 2.5 turns to tighten) That was the worst one, several others too a whole bunch of tightening too.
This leads me to say "Many RVers have a few screws loose" (The kind you tighten with a #2 square bit)