There are 2 separate 12v
systems. The running lights and the interior 12v
The running lights are grounded to the trailer skin and frame and and to theTow Vehicle through the 7 way connector. The interior 12v
system (OEM Univolt) runs through the converter. It is grounded by the 110 outlet. Newer converters are grounded to the trailer frame. As I understand it, all the interior 12v circuits are closed circuits and are generally not grounded to earth. There are several points where there are grounds to the trailer however. The 72 Service Manual shows the bathroom exhaust fan, galley roof light, water pump and scare light grounded to the trailer skin/ frame.
There are 2 ways to skin the cat. 1.The pos and neg heavy wires on the Intellipower connect to the fuse panel and the fuse panel is connected to the battery. 2. The Intellipower connects to the battery and then back to the fuse panel. I like #1 because it worked for me. I talked to someone who tried #2 and had problems, why I don't know. You can salvage the fuse panel from the Univolt or use a new one. There are lots of old threads on this. I know on the 9245 the 12v + and - cables connect to the bottom front of the unit.