Water Heater will not shut off

I have an older Atwood Water Heater on my fifth-wheel. It has not been shutting off when the water reaches temperature. The relief valve opens as it should and the hot water gushes out until I turn off the w. heater.

1. The water heater is working great on Gas and Electric. No problem in that part of the operation

2. Replaced the relief valve. That did not help. No shut off.

3. Just replaced the thermostat and the ECO this morning. This did not help. No shut off and the hot water begins to drip and then gushes out the relief valve.

What should be my next step. I am happy that the water heater works so well, but I need it to shut off when the proper temperature is reached.

I will appreciate any comments or suggestion