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

1. the house battery could get overcharged by higher voltages 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.
3. a b2b charger in parallel with an isolator could have the same backfeeding result

Isolator/relays/solenoids, etc, use mechanical or electrical means to combine the chassis and house 12v systems. These mechanical or electrical means will require a small amount power to operate.¹⁾

If the device's own power source is disconnected it will turn off and 12v systems will be separate again.²⁾ Using a High Voltage Disconnect to kill power to the device will stop the alternator from charging the house battery at a given voltage setpoint.

Shutting down the device will require different approaches, depending on where it gets the power to operate itself.

Purely Mechanical devices like solenoids and relays use 12v trigger or exciter voltage³⁾ to operate. They will either have 4 lugs or 3 lugs; in the latter case grounding is done through the base of the device.

Power to these devices is disrupted by interrupting power to the trigger+, which is one of the small lugs.⁴⁾

Voltage sensing relays (VSR) typically take power from the chassis battery and have a thin ground to complete the circuit.

Power to VSRs is disrupted by cutting the ground wire. One way to reestablish the ground (thereby turning the device on) is HVD –> relay NO contacts –> ground wire in/out

Some solid state isolators have a trigger or exciter input; drive with HVD as with solenoids and relays.

Three lug isolators without exciter input might not be usable with HVD.