Hi Joe's Folk!
I have used bleach before to remove smells from my washer... but I haven't tried it on a stinky drain. Maybe that could work? Especially if a food has rotted or something has molded over.
Also, I once saw a television show where a woman had a terrible smell coming from the drain in her bathroom. Obviously, it couldn't have been caused by food, and she called the plumber. He didn't find anything wrong. The smell persisted. Eventually, she took a chance and called an exterminator/pest removal service. They found a dead animal underneath her home, and the smell had risen up through her drain!
I highly doubt that is the problem in your case, but if you can't find any other solution after a while, the possibility is certainly not off the table! lol!