2018 Seneca 37FS 12v Issues

[Randyeng]

All 12v accessories work fine including bedroom reading lights and thermostat, refrigerator, light over kitchen sink, dining room light, and living room reading lights.
What doesn't work are the main living room lights, the light in the cab over area, bath light and fan, hallway lights and bedroom main light.
Checked all fuses, and all seemed ok. Checked voltage readings at fuse panel in bedroom, and the accessories that worked had voltage, those that didn't work had 0.0 vdc reading. What should the readings be, I was getting low readings.
I did fine a loose wire on the battery disconnect relay, pictured below, but once reconnected, no change.
Any ideas or suggestions?
Thanks in advance.

[SloPoke]

I would start to look for loose wires that go to the chassis... Known to be a chronic issue.

[RVermont]

As Slopoke mention the ground is a likely issue. These are connected to the rear of the chassis and I had a similar issue in my case lights would work until I turned on the fan then they would go out and some of the marker lights would glow. Found a group of wires hanging and not attached due to broken screw. Quick check is to run a long wire to a know clean ground on chassis and use that wire when checking the positive for 12vdc supply. If you are just probing a circuit using the existing wiring an open ground will give you a false indicator of lack of power.

In my 2018 unit 1/2 the wires are ground on the rear drivers side just in front of the rear cap. Basically laying on your back with the trailer hitch to you left shoulder there are several groups of wires connected under various screws. The other location is by the main coach door or below where your coach 12vdc push button disconnect is.

[Randyeng]

Thanks. Helpful stuff. I'll explore some more, and circle back with y'all.
Any idea what voltage reading should be at fuse panel?

[Randyeng]

Yep, that was the issue. Screw holding two groups of white ground wires had rusted and broken off.
Replaced and all is working good.
Thanks to all for the advice and solution.

[Rustynuts]

Pretty common issue, I had a group break off that caused my converter to stop charging the batteries. I upgraded that area with a Bluesea bus bar and crimped, heatshrink connectors on every wire after cutting them back an inch or so to cut off corrosion.