4 prong harness to pull a trailer behind my camper

I would put in 3 relays - coil powered off the TV's pigtail, and using the trailers battery to power the outgoing lights to the second trailer. put a fuse in each line (3) to the second plug. If anything goes wrong in the second trailer, it blows its fuse, and the remaining lights are unaffected as well as all the lights on the first trailer.
I check my lights at every hookup by turning on the headlights, and hitting the hazards. This runs all the lights on the trailer, and I can easily see if something is not working. I do that (walk around) at every stop where someone gets out. On my last truck I had a lighted 4 way plug, there were 3 small leds in the plug so you could see if the light problem was on the TV or trailer. I would use something similar of the back of your first trailer to simplify troubleshooting later on.

I ran wire from front of trailer to rear as i didnt want to try and fish wires down from taillights on trailer.

I bought 1 of these and works great.
About $30

http://www.tractorsupply.com/en/store/hopkinstrade%3B-adapter-7-pin-to-4

boogie_4wheel wrote:
Power lights on 2nd trailer from 1st trailer lights. No need to run wiring all the way to the TV.

If you can easily access the wiring at the tail of the 1st trailer, tap them (waterproof it) as far back as you can. 2nd choice would be to pull wiring to tongue of 1st trailer and access wiring at junction box. Each trailer will be different, so you're on your own.

This sounds like the best ways, the only other thing I would think about doing is to change the tail/brake lights out to LED,s on both trailers to minimize electoral draw on tow vehicle and first trailer's wiring.

Most newer tv have a 7 way plug and a 4 pin plug if so make the wires on the second trailer long enough to reach the tv

Power lights on 2nd trailer from 1st trailer lights. No need to run wiring all the way to the TV.

If you can easily access the wiring at the tail of the 1st trailer, tap them (waterproof it) as far back as you can. 2nd choice would be to pull wiring to tongue of 1st trailer and access wiring at junction box. Each trailer will be different, so you're on your own.

If both trailers have a "common bulb system" (one bulb for turn/tail/stop), you could use one of these kits connected at the lights of the trailer doing the towing:
Common bulb trailer connection kit.

I would run a new four conductor or three if you want to use the frame as a ground, to the junction box at the end of the ambilical cord. This way you do not have to play with wiring to your rig's lights. It should be fairly easy to route if you do not have an underbelly covering or enclosed frame.