Bypassing the Amp gauge - Question about the MAD Electrical method

What you propose is the "MAD Bypass" (search). Connect the 2 ammeter wires together, but don't leave them hanging w/ exposed copper. Most people put both wires on a single ammeter terminal stud. You already have the 2 big ALT and BAT wires to feed the cabin. No need for a new wire. They will no longer carry the large alternator current.