Sewer smell when using exhaust fans

We do use grey tank treatment from time to time and usually based on what food we're fixing....fish or something that would get a funky odor quickly from the wash water. We always leave the grey tank closed. If we are find that the tank needs to be dumped sooner rather than later, DH will dump it and then close up again. We've never had an odor problem and, while I had never considered it, the odor coming from the sewer itself makes sense to me. Suggest you try that first. Don't know that I'd put ammonia into the plumbing in case it's not compatible with some of the lines, etc. But a healthy dose of baking soda certainly would not hurt anything. Dump some in - maybe a box per drain - add some water and let sit, then rinse well. Couldn't hurt. Good luck.