Follow Up: Shutdown Problem Resolved
Thank you all for the suggestions and ideas.
The problem was resolved by replacing the Chassis Battery Shutoff switch.
The plastic stops in the old switch had failed. Could have been from turning too many times or trying to switch it the wrong way. Once the stops had worn away the switch could move ever so slightly and break contact. When I removed the switch, it literally fell apart in my hands.
I replaced with a heavier duty marine switch. Probably overkill but it is unlikely to fail.