Hi,
First, turn off the water heater. Then after it has cooled off (about 6 hours), turn off the pump (or turn off shore water), trip the relief valve, and turn on a hot water tap. This will reestablish the air bubble that is supposed to be at the top of the tank. Now turn the water heater on again. If that does not solve the problem then replace the pressure relief valve.
Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.