Also be aware that those lights are not brake lights, they are marker or running lights. They are basically wired around the top of the coach in a string, along with the three amber ones in the front, and most likely an amber on each front corner and red on each rear corner. (I don't know the specific placement on your coach, but that is a standard layout of marker lights).
It is possible that those lights are wired with a power lead only, and ground may come from the shell or a short wire to a shared ground point near the group of lights. The wire(s) from all those marker lights eventually ends up on pin #3, the top right slot as you look at the end of the trailer connection cord that plugs into the tow vehicle. When you turn on the running/parking/marker lights on the tow vehicle, all of those marker lights should come on. You can also put a small auto fuse (like a 5 amp) across pins #3 & #4 in that plug end, and if your coach 12v
system is working (battery installed and charged or converter working), that will power up the marker lights for troubleshooting.
Pay attention to rmkrum's post #2 above about finding that missing wire if at all possible...... Hope this helps.