DC Power Issue

Problem fixed! The ground wire had a bad connection somewhere. I never discovered where exactly, but I replaced the wire. This trailer has a wire from the negative battery terminal all the way to a buss bar next to the power converter. There is a separate wire from the buss bar to the trailer chassis. I replaced the (battery to buss bar) wire and everything works fine now.
I also cleaned up the chassis ground even though it looked good.
Thank you for all the detailed responses - big help!