You might have to consider checking for a sewer pipe break. I recently had a sewer smell emenating from my basement in a split level house. Turns out my sewer pipe running under my family room slab floor had rotted through. The slab had to be dug up in two places, the pipe repaired and the cement replaced. That did it.
In my case, I had to pay a sewer company to inspect the sewer pipes with a video camera setup. It was well worth it. That spotted the problem. If you are well vented and P traps are full, then you might have a leak somewhere.