Atwood Water Heater
Sounds like the thermostat. I would replace the thermostat and ECO valves first (about $15-18 a set). They are easy to replace, but make sure you put the foam insulation on that comes with them. If you have trouble doing it, send back a note or email me. I've replace mine and another friends and water heater works fine (similar problem to yours).
good luck and welcome to the forum....