My vehicle came without any trailer harness, I rigged it up myself. The charge line is a dedicated 8 AWG wire that runs from the battery to the 7-pin connector, through a relay and appropriate fuses.
I have 2 items wired directly to the trailer's battery (they bypass the master switch). First, the breakaway switch for the
brakes, so the
brakes will activate with breakaway switch pulled even if the battery master switch is OFF. Second, my
solar panels also bypass the master switch and will still recharge/maintain the battery when the trailer is parked with the battery switch at OFF. The
solar panel controller has its own switch that allows me to cut it off if I need to.