Sorry, I just re-read the posts. Is it possible the vent is clogged and not allowing air to get back into the tank? If tank is not full but sink won't drain, that may be your issue. Check the vent pipe on the roof to be sure there isn't a bird nest or small critter in there.
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Robert & Diana McNeal
2019 Montana Legacy 3791RD 20th Anniversary Edition
2014 F350 4x4 6.7L SRW
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