ENGINE BLOCK heater
only used in very low and long below freezing temps when the engine has not been cranked in a long while - only works on 120v shore or generator power, though.
If you 'accidentally' brush by the switch and turn it on, which I've done many times, I simply trip the breaker in the panel and don't worry about it. I've used our block heater MAYBE once in 100,000 miles over 6 years and in a LOT of cold weather. Then engine cranks just fine on it's own.
two Campers, two Electric cars
former diesel pusher traveler