On my rig, the 4/0 neg for the winch was grounded to the body. But it also has a 4/0 ground from the battery to the frame.
When i put the quick disconnect on my van for my inverter, i cut the plastic cover to the side connection on the battery a little so i could fit the end of the cables on behind. You can remove the little bolt on those and use a 3/8 bolt. I had to trim mine down some so it wouldn't bottom out. Way easier to tighten up than that 8mm head too.
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