Does it look like this?
It may be black instead of white. This pic is shot look up from the floor at the bottom side of a bathroom sink.
If it's real similar to this, it is a vacuum breaker/anti-siphon device on the black tank flush line.
It needs to be winterized like the rest of the water system. Some folks just blow it clear with air while others pump some RV antifreeze into it. Either will work just fine.
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'11 Cougar 326MKS loaded with mods
'12 Ford F250 SuperCab 6.7 PowerStroke Diesel
Amateur Radio: KD2IAT (146.520)
GMRS: WQPG808 (462.675 TPL 141.3)