Looking to put a aux pwr wire to the back of the new trailer I brought as it has a battery system in it for stand-alone wiring. how would one go about doing this?

Do it properly, add a high current Andreson Plug.

I would use a REDARC smart solenoid,50 amp circuit breaker,at least 6mm wire to an anderson plug with the earth wire attached to the frame,6mm twin cable to the battery in the trailer with a circuit breaker inline to protect the wiring from that battery to the car battery.Are you planning on going camping or setting up the trailer for work/fridge ?Cheers Kevman.