About a year ago we started noting a bad odor from the toilet. No amount of additives or cleaning would help. The bowl was holding water so I knew that the seal on the ball flush valve was okay. I even did multiple flushes of the black tank but the odor would still come back. I also checked the vent stack to make sure that it was open and it was okay.
Yesterday, I pulled the toilet up off of the floor and replaced the seal where the toilet bolts to the mounting flange. The old seal had a grove that fit onto the toilet and the seal was still attached to the bottom of the toilet when I lifted it from the flange. The new seal didn't have the groove and was flat on both sides. I added sealant to both sides of the gasket and to the mounting flange and the toilet. I also replaced the mounting bolts and nuts. Before tightening the nuts, the toilet was sitting about 3/8" above the floor because of the thickness of the gasket. I tightened both sides a little at a time to compress the gasket.
We now have absolutely no odor in the bathroom! It appears that the original grooved gasket had lost its seal and was allowing sewer gases to leak out of the tank.
Bill J., Lexington, KY 2006 Starcraft 2500RKS 25' Travel Trailer 2015 Ram 2500 Big Horn 6.7 Cummins.