There's a diagram of the 7-pin Bargman at the end of this link:
http://www.trailmanorowners.com/foru...light=bargeman
You will see that the extra wires are for 12V power, 12V auxilliary power and the TM electric brakes. You don't need to replace the wiring that already exists for the lighting, just get the new wiring added. It might be easier just to cut the four wire connector off and wire to a new Bargman connector, than to try to adapt.
You said you'll be towing a long way. You'll need a brake controller installed in your vehicle and maybe a WDH depending on your tow vehicle.
-Paul