We live in a mobile home in the woods and had a similar problem a couple of years ago. Several neighbors suggested we stop using antibacterial soaps, and reduce use of toilet bowl cleaners and chlorine bleach because they all kill the anaerobic microbes that work in your septic tank. They also told us to put one tablespoon of “baking” yeast down the toilet each month to feed the anaerobic microbes. After adding the yeast for several months we stopped having the odor. This is a very inexpensive solution. Hope this help you, too.