Have you contacted Freightliner to see if you can get a wiring diagram then contact Monaco to see if you can get a coach wiring diagram?
Your dealing with two separate 12vdc systems, the chassis and the coach.
It's a clean nicely laid out electrical compartment and not a rats nest you see on some of the older rigs. Wish I was closer, I'd love to sniff it out.
I can see your heading in the right direction with that test light in the first photo.
Hopefully a member with like coach will chime in with info.
Keep us posted.
Robert & Deb
07 Discovery 39v FWS Full Timers, Henniker NH
1K solar - 05 Jeep Grand Cherokee