Do your other switches there stay lit all the time as well? Do they light up only when the switch is on? That switch should act the same as the other switches in that location.
If they aren't acting the same, I'd test each wire to see which ones are hot all the time, and, as you suggested, are any hot only when the headlights are on.
If you don't have a wire hot only when the headlights are on, then there is no way the switch could be controlled that way. But you might be able to just have it on when the switch is on. That would require you to determine what switch terminal lights the switch, if it has one. (It could be connected to the hot wire inside the switch if always on. ) Off the MH, attach ground and then run a hot wire to other terminals until switch lights. Then use your wire that goes to the road light and jump to that terminal.
2014 Tiffin Allegro Bus 37AP