GMC4ME, I can virtually guarantee you have at least one, in the bathroom alongside the sink. It has to be as high up as it can be, to work properly. There was just room enough for me to unscrew mine to inspect and 'plug' it temporarily.
I too have three holding tanks because of the rear kitchen. I've never noted any odors from the kitchen sink, but maybe someday . . . I can imagine the kitchen sink smelling, because of the small scraps of food that end up going down the drain.
I'm still shocked at the notion that the grey water that serves only the bathroom (shower, sink) could smell so bad, but I guess it is what it is.
Thanks, Old Biscuit, for providing the picture. That's exactly what mine looks like.