Trailer wiring help

I have dealt with RV's and towing trucks for quite some time, and a lot of towing trucks have a battery charging (power source) circuits built into the wiring.
If I remember right it's a large black wire. (keep in mind that RV's use black for positive and white for grounds.)

Some vehicles use a ford type starter relay so it disconnects from the remote battery when the towing vehicle is not running, but activates the relay when started so it charges the remote battery.

My point is, that a lot of tow capable vehicles are wired for charging your trailer or RV batteries already. (through the existing trailer plug wiring)
I used to install the remote charging systems on vehicle that didn't have it.