Stinky water in bathroom sink

I agree with others that the odor is coming from the trap. Run a pint or so into a container and take it outside and give it a smell and see if it really is the tap water. If not tap water pour a cup or two of whatever you use in the black water tank and see if that cleans it up. Be sure to that the trap stays full of water. Frequently when underway the air moving by the vents will cause a vacuum and suck the traps dry. If this seems to be the case try a cheap vegetable oil in the offending trap before getting underway. The weight and thickness of an oil substance will not blow out under vacuum as easily as plain water. Check the caps on top of the vents and be sure they are present and accounted for. If not replace any that are missing.