I believe that this is where you tie together all of the 120v grounds, and tie them to the 120v ground pin from your power umbilicle. From what I've read, and I'm NOT the expert, problems occur quickly when the 120v ground and the 12v ground are tied together. The TT 120 gets it's ground back through the plug, whie the 12v grounds back to the chassis and battery.
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter