If you look in the sink cabinet you will see a black plastic vertical pipe in the drain plumbing. At the top of this pipe is a vent that opens and closes when you drain the sink. This vent is removable and often sticks open. When stuck open you get the odor. You can get a replacement at any Home Depot etc. or online. There is another vent in the kitchen sink.
I think that hiring an attorney in this matter is an excellent idea, for the attorney and his family anyway. You can google this vent application and learn why it is there. BTW grey tank odor can be worse than black tank odor.
By adding the bathroom sink water to the black tank it creates more fluidity making the complete draining when dumping more feasible.
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