/6 charge wire

Run to BATT+ or the starter relay stud, whichever is easier for you (same voltage). Read up on the "MAD Bypass" (gazillion posts). If drawing only 15A max from your battery, as you state, a 14 awg wire would be sufficient, so the massive 2 awg cable you installed is a waste of copper. The drop isn't in the factory wire, but in the bulkhead connectors, as you suspect.