One thing you might also check is the black tank flush vacuum breaker under the bathroom sink. When mine leaked, water showed up both where you describe and in the basement below.
I ended up cutting the pex and just coupling it below the bathroom in the underbelly. Then I put a backflow preventer on the hose inlet in the wet bay.
We also have a Fantastic Fan in the bathroom near 'A' in Mike's diagram above. Check that it is closed and leak free.
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Robert & Diana McNeal
2019 Montana Legacy 3791RD 20th Anniversary Edition
2014 F350 4x4 6.7L SRW
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