There are scenarios where one might want to prevent an isolator from combining the house and chassis 12v systems:

1. the house battery needs less voltage than lead-acid (lithium, for example)
2. the combined circuit can feed higher-than-optimal voltages back from solar charge controller to the chassis' electronics.

Isolator/relays/solenoids, etc, use mechanical or electrical means to combine the chassis and house 12v systems; this allows the alternator to charge the house batteries. These mechanical or electrical means require a small amount power to operate.

If this power is disconnected the isolator will turn off and 12v systems will be separate again.¹⁾ Using a High Voltage Disconnect to kill power to the isolator will effectively stop the alternator from charging the house battery at a given voltage setpoint.

by 12v trigger. Example: 12v source that's only active when the key is in the RUN position. These have 4 lugs: starting+, house+, trigger+, trigger-; or,

These usually have 3 lugs: starting+ in, house+ out, starting-. The starting- “ground” is only used to complete a small circuit so the relay can run itself. So it's usually very thin.