He only suggested that method for the trailer brakes as the brakes might be the only 12VDC system left. For the infrequent brake use, the aux 12v line from the tow vehicle ought to be able to handle the charging.
Exactly. Cargo trailer would already come with a small 12V battery to operate brake system, charged from the 7 pin. Intent would be to keep that in place and separate from the trailer interior battery bank.
Agreed. I have a 7x14 cargo trailer converted to a camper. The cargo trailer came with a small 12V battery in a box mounted to the side of the tongue. That battery is for the trailer's brakes. The 7-pin has a 12V line that controls the trailer's lights. I left all of that untouched. As part of my camper conversion I added LiFePO4 batteries, solar, etc. and a full electrical system completely independent from the trailer's original wiring and battery.
Good point, hadn't gotten to the point of considering how to power a backup camera. Where did you tap into the 12V power (brake lights, straight run to the tongue/brake battery, etc)? If you tapped into brake lights, does it only have power when brakes are activated? While I would use a 48V to 12V converter inside the trailer to supply 12V power, I wouldn't want to have to turn that converter on every time I want to back up.
I tapped into the trailer wires that are running above the rear doors. One of the wires is auxiliary power and one is ground. Those are the two you want to use assuming you want the camera to have power any time the 7-pin connector is plugged into the tow vehicle.
Your 48V->12V buck converter would not interact at all with the trailer wiring so it would be irrelevant if you connect the camera to the trailer wiring.
Many rigs have a running light at top center that people tap into. Just means having your running lights on 24hr which is a low amperage draw and not a bad thing for safety reasons anyway.
