It's usually just a micro-switch with an arm that sticks out far enough for the antenna frame to push it when down. It will be a normally open or normally closed type switch and should be available from many supply stores, e.g. radio shack.
Chances are the switch is fine and you just have a bent wire arm on the switch or maybe the mount bracket for the switch came loose. Or maybe a loose or corroded wire.
What brand of antenna is it? Braund 7 Concord usually have the switch, while Winegard's usually do not. Any brand could be equipped with one by the RV manufacturer, though.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL