RTS - this is a thread similar to your issue. Consider it a point of reference.
https://www.tiffinrvnetwork.com/foru...f=120&t=111833
Also - did Mr Harris suggest an alternate approach to the hard disconnect?
Seems like the disconnect should work if you waited until the after shutdown function was complete before you activated the disconnect. But that is an opinion unsupported by any facts. It would be where I would start my investigation.
Our chassis battery does not charge on shore power until the battery voltage drops down to about 12.2volts. Then it recharges the bank to 12.7volts. The converter is multi-staged and goes into float when heavy charging is not required. So, it would be my expectation that the converter would keep both banks charged if we had power when in storage. We do not. Our chassis battery does not go flat in 4-5 days in storage with the OEM disconnect in use. With the chassis disconnect not in use, it would.
Either your rig is wired in a different way than ours or something is not functioning correctly.
Possibility - there were some 2020 ISV 5.0 power coaches. If yours is one of those, it may be wired with an alternate configuration. In any case, I think Tiffin Customer Service is your place to start. May require a local tech to do the troubleshooting they suggest if you do not have that capability. Back to Gary Harris maybe.