Two posibilities, If you have a load shedding energery manager like the one photoed above.. Suspect #1
My story (I do not have one of those) Sometimes front A/C worked, sometimes rear sometimes both..... Finally got to trouble shooting.
First stop: The power distribution (Breaker) box (Also the last stop)
Several screws which hold the wires in the clamps were loose, like over two turns loose on one of them... Yup, that was the problem..
I keep saying many RVers have a few screws loose.. In my rig those take a #2 Square Bit.