Welcome to the forum!
1. It probably would be a good idea to run a "separate" 10 AWG ground wire to the 7 pin connector. The OEM wire is smaller because 2 tail lights and a couple of clearance lights don't draw that much current, the flat four isn't meant for electric brakes so Honda sized the wire accordingly. Electric brakes would draw more current so you would need a larger ground conductor. Make sure you attach your breakaway switch rip cord to the Ody.
2. You can leave the center pin open. You would have to splice it into the wire leading to one of the backup lights on the Ody if you wanted to use it. My 23B doesn't have backup lights and the wire is just stubbed off inside the coach.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.