simple things first
i would check and make sure the by pass vavles are not open or partially open, this would allow water to by pass the water heater and is used to winterize the water system
second bleed all the air from your tank just to be sure the air is out
you can bleed it by manually opening the upper pressure releif valve and hold it open till water runs out of it, either by using your fressh water pump or hooking up shore water
hope it helps
Living in the Piney Woods of Texas.