My '76 Sovereign
has had intermittent problems with the running lights. My trailer is very updated, but has never experienced a complete frame off rebuild. Therefore, the wiring in the walls has become a somewhat confusing mixture of old and new.
After checking the usual suspects: umbilical cord and connections, fuses, breakers, bulbs, etc., your problem is pretty much narrowed down to a short somewhere. It's like a string of Christmas lights; they still work if one bulb goes bad, but a short along the loop of lights shuts them all down.
To narrow down the location of the short, you can take the covers off the lights, and use a long piece of wire to "jump" the circuit from one light to the next. When they light up, you'll will know the short is somewhere between those two lights. If you possess a wiring diagram for your model, you can make an educated guess where your short may be. Or if you remember doing work or driving a screw into the wall in a certain spot...