You can add an external power feed wire, but you must switch both the hot and neutral wires for the a/c. Don't try it with just the hot. You do not want to risk having two different circuits cross feeding in any way.
In reality, the ground wire ought to be switched too, since the external source you use may have a different ground reference than the rest of the RV power system.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL