Why would there be a foul odor coming from under the bathroom sink as well as from the washer and dryer if the washer and dryer share a common wall with the bathroom?

It's not helpful to answer with a question but...what type of structure is the wall and floor made up of...A leaking pipe can cause serious odor due to mildew and other biological concerns, especially in a wood framed wall and floor system...Or...if you have a manufactured (mobile) home they are sometimes equipped with an "auto-vent" which is a mechanical device that is meant to allow air into your drainage system but not allow sewer gas to enter the dwelling...if it is working properly.