Stop in for a cup of coffee

My cousin say he has seen a cut off relay that cuts out when the voltage drops under 13 volts. Like to use between a car and the camper caravan. You start the engine, the voltage goes up past 13 volts, and the power to the 12 volt battery in the camper caravan is charged, then you stop the engine, and the voltage at some point drops under 13 volts, and you use power of the camper caravan battery but nothing from the truck battery since it has disconnected. Has anyone heard about such a thing ? I find someone mentioning that Bosch are supposed to have it, but can not find the actual relay.

Bill
It takes some planning,a second battery, relay,circuit breakers and a starter solenoid. battery is for starter only, charges while ignition is on. And a push button.
Will start every time.