A 7-pin Bargman, heavy duty flasher, 5 amp fuse, single pole / double throw toggle switch, and a box to put it in is all you need ...
Trailer Lights Dongle (3 pics)
In the situation described by the OP a G24 deep cycle battery would provide sufficient power to flash the lights on a typical travel trailer for days. I use my trailer lights dongle every time I back our trailer from the street and on to our driveway, and should I ever be forced to leave the trailer on the side of the road I have the ability to flash ALL the trailer lights for maximum visibility for as long as may be necessary.
