Like many others, I think you're making a mountain out of a molehill. I have built 2 testers; one for my camper which utilizes the on-board battery of the camper to run the lights via the 7 pin RV plug. I put my back up lights on the center pin. In the attached drawing, I mention a single pole-double throw switch, I used the same type switch, but these are spring loaded momentary switches; either type will work, it's a matter of choice, cost, and availability. I had the switches already, so thats what I used.
The other was a simple light tester with small clearance lights that act as my camper's tail/running lights & turn/brake lights. I mounted the clearance lights on an old metal license type plate, with a pigtail long enough to let it hang on the inside of my tail gate so I can see the lights working as I push the brakes etc. The black wires you see running to each light are the power wires from the 7 pin RV Plug; the lights are grounded through the metal plate. You may notice a bulb for the trailer brakes. This won't work! The signal to the brakes is different, and I don't know how to make that part function.
These tools might have cost me $10-15 a few years ago, but I would guess them at $50-60 for the pair today.
I issue the usual disclaimer, I'm not responsible if you burn your fingers, or torch your vehicle due to your inability to work with DC current, blah blah blah.
Last edited by Stale Trooper; 01-29-2012 at 03:17 PM.
Reason: Edit photos & add diagram