There are RVs that are plumbed that way, the purpose being to add more liquid to the black tank so it will drain better.
In my grandmothers house the plumbing was added "after the fact." So the men folk would line up at the kitchen sink to shave and freshen up in the morning, as there was no heat in the bathroom, which was a walled in part of the back porch! Taking a clue from my uncles, I'd just shave in the kitchen or outdoors when dry camping.
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2008 (or is it 2009?) Rockwood 8280SS - 2007 F-150
Lot's of mostly Kenwood radios