[Tommy]

Ruble -
I put a 12v battery in a marine battery box inside my trailer and wired it to draw current from the truck's system through the trailer connection. As I recall, I ran a dedicated, heavy gage power wire from the truck battery to that pole on the trailer connection. I did that because I expected the battery to occasionally draw a high load during recharge and I didn't want to overload an ordinary circuit. ... As for charging the car while towing the trailer - If you have a battery in the trailer, all you'd need to charge the car would be a long set of jumper cables. The truck electrical system would be providing power to the trailer battery and the car.

If you have electric brakes, there is likely a small battery mounted on the tongue to provide emergency electrical power should the trailer come loose from the tow vehicle. On my trailer, that battery does not provide power to the interior lights or anything else but the brakes.