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Hmmm! Check to see if there is a "P" trap under the sink near the drain. There is supposed to be standing water in the trap to keep odors from coming back up. Is the odor more prominent after the unit has been standing without use for a while? If so, then the water is evaporating letting odors up through the pipe line. But, I would assume the same would happen to the other drains.
You could try putting a plug stopper in the sink and run water in the bowl before pulling it to allow the trap to fill. Also, a good cleaning of the holding tank may be in order.
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