Most campers have auxiliary generators. If broken down on the road these can be used to warn oncoming traffic or provide illumination for repairs if you have several inexpensive clip-on 110 volt lamps and long extension cords. You can use 100 watt spotlights or colored red bulbs with a "button" christmas flasher in the socket.
The lights can slip on the rear bumper.
This will work as well as a road flare on a rain-swept night with low visibility.
Most campers have rear windows. If you leave the internal lights on and have a roll of red translucent plastic to place across the rear window it will provide an additional warning to approaching traffic.