Have you tried disconnecting the main feed from the engine battery at B1 and taken the voltages at the B1 terminal and at the disconnected wire, both with the engine off and when it is running? I think this would help decide whether the problem is in the DS300 or on the engine battery wiring. You will need to be careful when doing this, temporarily disconnecting both batteries while removing the connection would be safest until you have tied the wire safely away from coming into contact with anything else.