Smell in bathroom, Duplicate thread

I have a double wide and the back bathroom has had a smell for a long time. The two sinks have avv valves on them then there's a 1 1/2" vent going out thur the roof for the toliet and tub. I went up on roof and had someone flush toliet and u can hear the water flowing and I have had a plumber out and all the sewer pipes are clean under the house. Was Woundering if anybody knew where the smell is coming from. Thanls