Ran across this -- see the note at the very bottom: https://www.ramtrucks.com/assets/bbg/pdf/2016/docs/cc/dddpatbcw.pdf
The system I have was originally on a Dodge truck. It is now on my Toyota truck. The system was an after-market installation I purchased from my trailer dealer. My trailer is up at our church getting loaded for its next trip to southern WV with flood relief supplies, so I can't see who made the system, and my feeble brain doesn't remember who made it. But, moving it from the Dodge to the Toyota took less than 30 minutes. I have the round hitch-mounted power connector. The battery box mounted on the trailer tongue is part of the system that receives wireless signals from the electronic sensor mounted under the dashboard to the left of my steering wheel. I can set the level of braking control through its attachment to the brake pedal, and I can control the trailer breaking manually. The brakes do not really draw power from the truck battery, except to keep the deep discharge battery charged. The battery/controller box is isolated from the truck battery through a steering diode like those used in RVs to charge the coach batteries from the engine charging system. The battery is sealed and about the size of a lawn mower battery. It has its own charge controller and has a switch to open the circuit. I remove the box when the trailer is not going to be used a several weeks and bring it into the house for trickle/maintenance charging. Mine is wired to the tail light circuit. I always drive with my headlights/tailights turned on, so their is charging current to the trailer battery whenever I'm driving. For my SOTA/BOTA work I use my truck or my side x side UTV, but my power source is a battery jumper box. I'm usually running either my old Argonaut 505 at about 3-5 watts, plus a laptop on its own battery. On receive it only draws almost nothing. Or, I'm using a 2 meter/440 MHz handheld. I can go a whole afternoon and not run out of power. If I do, I can start the engine on either vehicle and start to recharge the battery box. Ken K8KJG
Most are 30 amp #10 wire on the rv charging, note some trucks even with the big trailer plug may not be hookup at the main fuse block and may not have a fuse in place
A minor point... There should not be any battery charging done through the brake line. Using a wireless controller, the brakes are drawing power from the trailer battery. It would be foolish to wire a tow vehicle so that constant 12vdc is on the brake pin. That mean that the tow vehicle could not tow any trailer with standard wiring. There is a separate pin in the 7 pin connector for trailer charge which is constant 12vdc. That keeps the trailer battery charged. I would never custom wire a 7 pin trailer connector in deviation from the industry standard. It would be a recipe for a catastrophe.
Could be worse. I had to install a factory tow harness in my Dodge (Mercedes) Sprinter, and it required taking the dash apart, tapping into the brake light switch, some wiring under the driver seat and hood, and bundling up an extra 4 feet of wire that was unused on the shorter vans. One boo-boo they made was connecting the trailer brake wire straight up to the brake light switch. They left the wire disconnected on later versions, and later deleted it from the harness completely. I think I have only used it for small trailers, and the 12v line gets used for backup spotlights and to power the flashing stop lights on the crash bar in the back.
Pin 4 is the 12volt its dedicated for 12 volt RV Trailer charging car are auxiliary batteries on any type of trailer
That's the funny (odd) thing about travel trailers and fifth wheelers. For some unknown reason, that industry has settled on white being ground and black being hot. Unfortunately, that logic has extended to the tow vehicle side of the standard 7 pin trailer connectors as well.
