All your tests, connections and bypass wires should be under your bed up front.
As pointed out, all leads are focusing on your 12v
switch and that same bundle of wires at the switch are also at the solenoid switch that isolates your batteries to your coach.
Start at your batteries where they enter the coach, under your bed. This is where your solar hooks into the system, where the power comes out to the intrior of the coach via the 12v
isolator switch. The same five wire bundle you have at the switch near your rear entry door comes out under your bed. Test here to see if it's the switch button or the contact battery switch itself, or just connections.
Remember that testing with bypass wires are ok for a short time during trouble shooting, fix with proper wire size and components.
If you are in the SoCal area, RV supply and AS dealers are fairly close.
Good luck, and ditto on the electrical knowledge, 12v
and 110v , add some solar and an inverter, gotta be fun. I have the same componets in our rig.