HI,
I did the same thing. I installed a 4 wire pigtail, just like the tow dolly plugs into on the RV on my tow dolly wiring harness. Then installed the remaining 4 feet of wire to my minivan, and extended that wire to the rear bumper area inside the car, protected from the weather. (I used 4 wire thermostat wire, I have hundreds of feet of the 18 gauge wire sitting around).
At the taillight, I removed the bulb, and discovered the wiring. I think it was green on one side, yellow on the other. I installed a diode made for wiring towed vehicles (kit from Roadmaster, sold at Camping World) with 2 inputs and one output going to the lights. So the output went to the minivan light, one input was the wire I had cut, the other came from the RV wiring via the tow dolly connection I described.
I also installed some 3/8" eye bolts to the underside of the van, near the front bumper. On the dolly, I installed some chain and clips rated for 5,000 pounds or something like that. So the van was safety chained to the dolly, and the dolly to the RV hitch.
After each tow, I would stop in about 300' and tighten the straps a little. After a couple of miles, I would pull into a parking lot, and just check things over again. Usually after about 50 miles, for whatever reason, one of the straps would normally work it's way a little to loose. Never did figure out why, just happy that it never came all the way loose!
I also made a 7 step hitch up process, as long as I counted all 7 steps, I knew everything was good to go. But forget a step (like the third time I towed) and the safety pin that keeps the tilt ramp from tilting while driving might get forgotten. .. Bummer.. .
Have fun camping!
Fred.