I will weigh in a little
I have the Atwood 10gal DSI gas/electric, motoraid water heater.
The 120volt side is separate from the gas side.
On the electric side, I have a 120 VAC remote switch on my EMS panel. This sends AC to the 120 VAC switch located on the back of the heater (which should always be turned on). The AC switch on the back is used in installations where a remote switch is not used. The hot AC side then goes to the thermastat, then to the ECO stat, then to the heating element, then to neutral, completing the circuit.
The ECO is a manually resettable unit. When you look at it, there is a small white button located between the two electrical wire connections. Pushing this in resets the ECO.
It is in a very hard place to reach, at least in my install.
The tstat, ECO & element are all covered by a metal cover & glass insulation, similar to a household water heater, but there is a small hole in the cover over the ECO which allows you to insert an object like a pencil eraser to push in the button.
I plan to remove my wh when we get back home from this place, in order to fix my AUX coach heater, which is located behind the wh. At that time I will check & see if I can replace the ECO, or at least make it easier to reset.