As you can see from the diagram, very simple system.
OK, here is a thought.
The thermostat assembly is in 3 parts but if any of the 3 is bad it won't work and needs to be replaced.
Top part: HIGH LIMIT switch - Normally closed and pops open when over heated shutting off power to the element. Can be rest by pressing in on the rubber covered button allowing current to leave the bellows switch via the fusible link wire.
Middle Part: The Fusible wire - When it is overloaded the link wire will melt and open the circuit turning off the power to the element.
Bottom Part: - The LOW LIMIT switch - Normally OPEN when water is hot and closes when water is cold (to allow the heater to come on). THIS switch does NOT have a reset. If this switch is bad the entire thermostat must be replaced.
(You can try tapping it with a screwdriver handle to see if it can be unstuck and close properly.) I would be ordering a new thermostat in any case. If it unsticks; it will eventually fail again.
Suburban 232306 RV Water Heater Thermostat
PS easy to change; make sure the circuit breaker is tripped open before you start.
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