I do not believe Workhorse does it this way but I have seen CARS where power to the dash is tapped off at the starter BATTERY terminal (That's the big one).
Now,,, You say you disconnected the battery..
On my WOrkhorse there were 3 wires hooked to the POSITIVE terminal and one negative
Jacks
Starter
Intelletec Battery Control Center (Ignition, dash, everything NOT the starter or Jacks)
Due to the use of the WRONG type of metal this connection was a major issue.. So I got a SHORT battery cable (*Actually a repair cable) and moved those wrong metal connections a few inches away from the battery.. Also reduced to just one wire to the battery (a proper battery wire) Just so you know.
I suspect that is where the issue is. Missed a wire at the battery
Option 2.
IF unit was connected to shore power. the BCC wire would still have been HOT, possible issue with the BCC,
