Ours has a fairly complicated EMS system. If I plug in and switch on both A/C's, it might be 5 minutes before the first one comes on, and 10 after that for the second. It usually favors the bedroom, oddly...
It does have a display that shows me what it is doing if it starts to shed loads.
Can you get the rear ac to run with nothing else on?
Is your EMS display showing 50 amp or genny power and everything on?
The HVAC system, including the circuit boards on the A/C units and the thermostat, all run on 12 volts and interconnect with what looks like phone wires. (Yes, the A/C unit receive, and won't function without, 12volts.)
I'd reboot the entire system, perhaps going as far as physically removing all power to the coach. (unplug and flip the battery disconnect, or disconnect the neg battery lead if you don't have a real physical disconnect switch)
Then power back up and try the rear a/c alone first.
