Suggest you install a separate box between the main breaker box and your rear A/C unit. then equip that box with a "transfer switch" rated at 20amps. A transfer switch is basically a double pole, double throw switch.
You then feed one side of the transfer switch from the main breaker box, and connect the other side to your (separate) 30 amp shore power cord.
Then you can safely, switch your rear A/C between the main motorhome power, and (separate) 30amp shore power.
Make sure you run the proper ground wires when you do this, it keeps you from doing the 60 cycle dance.
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