I agree with just there. In fact I've done that one of my air conditioners has the OPTION to be operated 100% independent of the rest of the rig.
There used to be a company (PSRV or Power Solutions RV) that made a device to do this.
you need a single breaker breaker or breaker/switch box 20 amp
Some 12 ga wire. You may need a junction box a 20 amp inlet and outlet
Unhook existing 12ga line to roof from breaker box and run to new breaker box (NOTE if necessary go to a junttion box and extend to net breaker box
12 volt line from new breaker box (inlet side) to 20 amp inlet or plug (plug in a basement compartment or inlet mounted in a box in same
Eventually you will run 12 (or 10) ga to a pigtail coming out of the same box you mount the inlet in but that comes later..
now run a 12 ga or heavier cord to your shore 20 amp outlet
When you are done.. If on a 50 amp site. simply plug the Pigtail outlet onto the Inlet and Enjoy... If on a 30. pull plug off inlet and use 12ga cord to 20 amp outlet.