Does your rig have an intellitec EMS?
(my 08 monaco does...)
Does the ems show a light for rear A/C?
as well as showing 30 or 50 amp shore power or genny running?
check the rear a/c 20 amp breaker in your A/C panel if the ems indicates it should be on.
There is a delay in this system, almost 3 minutes from when the EMS indicates power ison until the A/C's will actually start.
Mine might start either first, then wait a few before starting the second.
Either of my A/C's sometime blow their breaker in the 120v circuit panel, everything else looks normal but the compressor never fires off (fans will)