Hi,
Hope this helps solve your problem. A friend of mind had a similar problem with there basement bathroom that they hadn’t used in months. The smell was awful. It turned out to be the U Trap drain in the bathroom floor had dried out. They poured a couple of buckets of water down there to re-fill the trap to stop the raw sewage smell coming though. In know time the smell had gone. Also if you haven’t used your shower in a long time pour water down there as well.
What really helps to get your drain pipes flushed clean is to bring your garden hose though the bathroom window and turn it on full strength down the traps and into the toilet. Don’t forget to put some old towels on the floor to prevent water escaping out of the door.
Lets hope it’s a simple as that.