Negative/Ground wire
Our first trailer had both the positive and negative 12 volt wires running from the battery to the combination distribution/converter panel. The negative wire was also grounded to the frame. On the ...
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nothing wrong with adding the appropriatge wire size for the ground as long as you keep the existing frame grounding. I'd recomend updating the frame ground with a 1/4 self tapping or threaded bolt & star washer.
In reality the extra run isn't going to do much for you.
The frame is a lower resistance path than any reasonable sized wire is, and more than likely all or most of the 12V fused runs ground to the frame as well so you still need the frame ground. All the extra wire will do is give a ground path from the battery to the converter, yes, it is in parallel with the frame ground but the frame ground is much lower resistance.
In reality the extra run isn't going to do much for you.
The frame is a lower resistance path than any reasonable sized wire is, and more than likely all or most of the 12V fused runs ground to the frame as well so you still need the frame ground. All the extra wire will do is give a ground path from the battery to the converter, yes, it is in parallel with the frame ground but the frame ground is much lower resistance.
