Occasionally we get a strong sewer smell from all 3 toilets in the house.

We smell the tanks, and no odors are coming from any of the tanks. It seems to go away in a day or two. It happens randomly about 3 or 4 times a year.

The seals are good so I'm wondering if it is possibly a vent issue.

Thanks.

Could be a vent or could be a bad wax seal under the toilet but all 3 at once sound pretty unusual. Are you 99% sure it is sewer gas?