Sounds like the pressure regulator or pressure relief valve at the fresh water intake.
Do you keep the intake covered when not in use. If not, you may have mud dauber problems.
I would pick up a fitting that will screw into the drain plug opening (1/2" NPT) with a hose connection on the other end. Then connect a garden hose, pressurize the system and see if you have good pressure. If so, I would remove the old hardware (Pressure reg and relief valve) and replace with a section of pipe. Then use an external regulator.
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