Under each of your sinks there is a vent connected to your drain pipe. This vent opens/closes, (as needed) to keep the draining water flowing and to keep a vacuum from forming in the gray water pipe. Unfortunately, the smell escapes from the vent when the air pressure inside of the TT is less than the outside air pressure (ceiling fan running with no window open, and possibly the heating/AC system running). This seems to be a flaw in the design of the vent. The best thing to do is keep a window cracked if the ceiling fan is on. I got tired of the daily odor and taped the bathroom sink pipe vent closed. There is no harm caused by this, and there is no difference in the rate of water draining from the sink or bath tub, or when draining the GRAY water tank. I know that this is not a recommended fix, but since then, I have not had an odor problem. It works for me. If the DW is happy, so I am happy!
Don
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