The outside sink is indeed lower than the gray water tank on my coach, as the gray water tank is above the fuel tank. That is why there is a macerater pump on outside sink to pump water up into the gray water tank. On my coach, some nice sole at the factory put a 1.5 inch one way flapper valve in the line to the gray water tank. It is the same flapper valve under the shower drain I would suppose yours could be omitted, sticking or otherwise inoperative. You could try filling the outside sink with hot soapy water a couple times and pumping it out with the macerater and see if it clears your problem
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Jim & Roy Davis
2016 Hurricane 31S
1961 Rampside in tow
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