There’s no need to change the trailer wiring, just make up a new pigtail using a 7 way plug, a 4 way vehicle end plug and a 2 way polarized plug set. The 2 way will be heavier gauge wire and give you a dedicated charge and ground wires.
Be sure to connect the 2 way such that the positive side of the trailer end is the shielded one.
If you only ever want a trickle charge and take longer drives you can always just run a jumper wire from the marker light circuit to the battery and drive with your lights on.
