There's often a thermostat, and an over-temp control... and a separate heating
In our system, the thermostat and over-temp switch (sold as a pair) need periodic (not often) replacement. Usually the heating
elements are more robust, as long as they haven't been mis-used (run while dry, etc.).
If you check your water heater
manual, you may find a diagram of parts
. Once found, you can usually check for power at the thermostat/OT switch. Sometimes it'll be visually obvious... as in a separate wire...
The thermostats/OT switches are often available at RV dealers, too, if you have to shop for one.