I assume this technology has not changed in 20 years! If so, the thermostat and ECO touches the tank. It's located behind the black foam tape square if you still have one.
It's just a normally closed (NC) switch that opens at ~145F. So maybe if it's not making contact, or if the contact is oxidized, which is probably the problem, then you just need to clean it with some fine black sand paper (1000 grit), or you may need to replace the thermostat. (ECO too while you are at it.)
I carry a spare. And don't forget to use a protection diode. (See last picture.)
==> When installing the ECO and T-stat, be sure to position the spring so it mates against the plastic protection piece (works like a dielectric plate) and does not SHORT out. ...You sometime need to bend the metal tabs just a tad so you can twist everything in place.
==> If you find your hot water pulses out of the shower after camping for 2 to 3+ weeks, changes are you lost your "air gap" in the tank. To fix this, just turn off the water pressure in the hose, turn off your pump, and open the emergency overflow valve.