In generator equipped RVs that didn't have an auto transfer switch, a simple outlet was installed. The main power panel led to a male 30 amp outlet (or 50 amp if needed) The shore cord was modified to the RV end with a female plug. A cord from the generator also ends in a female 30 amp plug. Which ever plug is in the outlet, that's where the power comes from.
Since only one can be plugged in at a time, no phase issues. When leaving a campsite, always unplug the shore cord and plug in the generator in case you need it while driving or stopping for lunch.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI