So the replacement valve doesn't work any better and it still leaks under pump pressure. It sounds like there is some kind of debris in the water system that is getting stuck in the valve. Either that or when you installed the new valve you did not get it seated correctly.
since I'm guessing you have debris in the valve I doubt a Thetford would work any better.
Do you possibly have enough hose length to disconnect the fill pipe from the valve, wrap it in a coffee filter and put it in a dry bucket. Have someone turn on the water pump while you hold the hose end in the bucket. Count to 30 and turn the pump back off. Now check the coffee filter and see if there is debris stuck in it.
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2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup