With everything you’ve done so far it seems clear thermal expansion is your problem. As the water heats it expands in the water heater tank. Normally water heaters are designed to expand into the incoming cold water line. Our RVs are a closed water system. The check valve in the incoming city water connection is what makes it a closed system. Since the expanding hot water cannot expand past the incoming check valve, the pressure builds up in the water heater. Once it gets to 150 psi your T & P valve leaks. Perhaps the water heater thermostat runs a little longer than it should causing pressure to build. jerryjay11 and Thystra gave you the correct answer, an expansion tank. Sometimes looking at a posters profile will give you some insight into to their answers. jerryjay11 has HVAC an appliance experiance, giving more weight to his answer. One other thing. The air chamber at the top of the water heater tank, recommended by Suburban, only works until you turn on a hot water faucet. As soon as the hot water is used the air is forced out of the tank. You’d have to create the air chamber again every time you use hot water. Not very practical.
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