mine is the same ,I use the same setup now ,both plugs .The larger is for the
trailer brakes ,lights, brake /turn signals.The other is for the 12volt power
and ground wires ,ground from tv frame to trailer ,the 12volt wire from tv charging to trailer spliced with 10 gauge wire to battery in front box .the 12volt lead is in the 4 pin connector at
trailer silver and brass pins are both 12volt (the spliced wires)As you noted,the top pins are grounds (4 pin connector)The secondary plug from trailer wire is the 12volt source from 4 pin connector ,from tv (not at the large connector 7 way) inside wire used from the 4 way plug yes ! that wire
connects the 12 volt
tv charge to the battery.The inside wire to the far right
is for the grover 12volt air pump (switched from the wall switch above by front window)the black with red twist connector on it.the one next to it goes up to the wall switch also, 12 volt
.Both in one wall grommet at right .