1971/72 Terry Travel Trailer 24ft

What I didn't realize whe I started with a similar situation was that the skin of the trailer is actually the ground wire in many cases. What you should do is take off the reflector of each light and connect to your truck to the frame of each light in turn, and use that as your ground for checking. Then see if either filament works when you try running lights and brake lights. It's actually prety simple and you can get easy help if you pm me back.