Need help with my sketchy battery cable situation

there is a thin black ground wire coming from the taped up part of the + cable, which is bolted to the trailer frame. Why would the + cable be grounded?

Seems to me that someone has set trailer up to work on a positive grounding system. It will be a job to trace the wires all the way back to the converter or fuse panel to see what is going on with the wiring and maybe why you have a wire from the positive side of battery to frame.