I would be happy to help!
It sounds like you have a bad ground, as OL on a multimeter means Open Loop which is no continuity. I would move the ground wire to a different location that is corrosion and paint free metal. I would highly doubt that the center market light on your 2018 Prime Time Fury Toy Hauler is not 12v, as I am unaware of any trailer manufacturers using 6 volt lights anymore.
Once you relocate the ground to a better surface, let me know if the camera starts working.
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