Have you tested the coach batteries for state of charge? If the battery enable/disable switch makes no difference, and even the detectors aren't reacting to it, your batteries maybe the problem. Or the disable/enable switch, or the battery isolator, or ? ? ?
If it isn't the batteries, it's connectivity somewhere. Your symptoms suggest dead, or very low, coach batteries, in my opinion. However, it could be as simple as a bad battery cable or battery clamp or lug at the coach batteries. Tug on each one of them (firmly, but carefully, don't short a loose + one), and see if any are loose or otherwise defective.
Lastly, what happens if you plug into shore power? If some stuff wakes up, note it, and the still dead stuff, and update the thread.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 JKU (sold @ ????)
2016 Sunstar 26HE, V10, 3V, 6 Speed (sold @ 4600 miles)
2002 Roadtrek C190P (sold @ 315,000kms)