One common arrangement is to have the emergency start switch be a SPDT switch. The common contact goes to the relay control line. The normally closed contact (i.e. when the switch is not pushed in) goes to some ignition switched power source in the chassis. The normally open contact goes to a house 12V circuit. The same relay is used both for emergency starting and as the battery isolator. Obviously in this case that relay needs to be rated for continuous duty and also capable of handling a short duration high current.
Sometimes the two systems are completely separate, too, and another relay or solid state power diode used to permit charging the house battery from the engine alternator.
