"Someone approached for help will never see the problem; however, anyone not approached for help will see the problem immediately."
Here is what I think:
The silver box is a 40 amp current limiter and the red wire on it goes to your power converter.
The black fuse holder is power to a large current draw item (like a slide, landing gear, or power front jack).
The black wire is almost certainly Battery Ground and should go from the battery to the frame directly.
The white wire should also go directly to the frame ground as I believe it is the ground wire for whatever the black fuse holder powers. It is VERY unusually for grounds to all go to the negative terminal, but I see no reason why they could not. I think the black; the white; and the green; all go to the negative terminal.
MISSING from the photos. The battery.
My money is on:
There is a wire still missing from the photo. A red wire from the battery to the silver box terminal with no wire attached. Look around.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW