I spent most of the week playing with my wiring and moving batteries/wires around, so this is fresh in my mind.
The negative side of the DC has a bus bar nearby the converter in my TT, it also happens to be the bus bar for where the AC side gets grounded to the frame.
The negative wire of the battery attaches to the frame at the tongue, and there are some negative wires that leave that terminal but without removing the underbelly I have no way of knowing if the negative wire that feeds the bus bar is full length, or taps into the frame nearby.
The converter will have a negative lead, it will attach to the DC distribution panel and there is another spot where the distribution panel will have a lug labeled batt (-), run that to the bus bar just like the AC ground.
The single light is fed off the + line from the 7 way, that is the line you will want to run to your battery (thru a 30A fuse) and then from the battery to the batt (+) connection on the DC distribution panel. Ground the frame/battery/7 way to the same spot on the frame and then based on location you can either ground the bus bar there or a closer spot from the frame.
Probably more than you where asking for...