That valve by the sink in the bathroom . . .
Got stopped at a campground a few days ago, and was assaulted by a really terrible smell in the bathroom. Cut to the chase: I found it was that valve that allows air to flow when the sink is drained. ...
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Replace the valve, you need to vent the drain for proper drainage. They're cheap at home centers; called an "air admitance valve" or "cheater vent". Under 10 bucks. Simple to do.
