You may be right about a siphoning problem. Even with a continous drip, which would ostensibly keep a leaking P-joint full and water-sealed, I am still occasionally getting a major sewer gas smell. This bathroom in on a concrete slab. The tub is apparantly offset from the P-joint so I can’t see downward ver far. Do you thinkg a plummer with some sort of orthoscopic device might be able to see what the problem is?