Trailer wire harness

There are numerous ways to approach this and I suppose there are some commercial inline kits that are a plug and play. The #4 pin on a 7 way is designated for 12V battery charging however that generally is for charging the small battery used for electric brake breakaway. I was not comfortable recharging a full size TC battery via the under dash 14/16ga wire. A depleted automotive/deep cycle battery as used in a TC can pull 10-15 amps. That is above the rated wire capacity for the distance. I'm figuring from alternator to the bumper on a crew cab F250 = 18' plus another 8' back to the TC inverter/charger = 26'= 8ga wire. But then that is just an opinion and I could be wrong. A 60W light bulb is only a .54 amp draw but through a VERY thin wire. Guess that is why it glows white hot. Just tried to help.

http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html