[JRTJH]

Pull the cover off the switch and look at the wires. Are the "solid copper ROMEX" or are they "multistrand copper" ???

If it's ROMEX, chances are very good that it is a 120 VAC circuit and you should focus on those type components. If it's multistrand, then it's probably a 12 volt circuit and typically is a "light fixture"....

Most trailers from that era had "scare lights" on the front of the sidewalls. Some only on the curb side, some on both road and curb sides. They are the 8" round "dish like" lights mounted high on the sidewall, usually just aft of the front clearance light.

NOTE: Those light fixtures are notorious for leaking and quite often have corrosion on the bulb socket, so they quite often do not work or do burn through bulbs.... So verify that every light works before just "flipping a switch and determining that the switch does not operate that light"....

Nothing is more "self defeating" than spending hours, days or even weeks troubleshooting to find that the first inclination that it controls the main door step light (under the trailer) is what the switch controls, but the bulb was burned out, so all those hours were spend "chasing your tail and getting nowhere"....

Check the fixture for 12 volts and check the bulb for continuity.