Is this a travel trailer or 5th wheel?
5th will have either a junction or wires just connected and wrapped up in king pin... Travel trailer should have a box on frame right behind propane tanks and battery usually. If you find the wires you can check the umbilical card for continuity for the running light. Do a Google search for the pin out. Remember the key and looking at front or rear.
Livin the dream... Retired!
TV 2016 F250 Crew Sterling Gray SB 6.7 PS airlift ultimate Air bags
2017 Eagle 321RSTS