You can skip part of this if you aren't really concerned about best practices for wiring AC devices.
What I would do if I wanted to accomplish the OP's circuit configuration is similar to what others have said, break out the AC supply to the air conditioner using a male plug and a 20 amp receptacle. One half of the receptacle would be powered by the original breaker in the RV's load center. The other half of the receptacle would be powered by a separate shore power cord, power to the receptacle interrupted by a disconnect switch with either another 20 amp circuit breaker or a fuse. Disconnect switches are cheap enough that placing circuit protection inside the RV in addition to the power source (pedestal, generator, home receptacle) would be a no-brainer for me. The OP may choose to install a breakout box with two receptacles so that an auxiliary heater or other device can be used without unplugging the air conditioner's male plug.