We have had problems with our toilet seals "leaking" gas but not water.
when we moved in to our trailer the old partical board sub floor had failed so that the toilet was reclining nicely. the Floor, insulation all stunk. It took months after i replaced the toilet for the smell to go away. After that depending on weather conditions we would get a whiff of septic. turns out when i replaced the toilet and put new floor down the was ring had missed a section no larger than about 1/4" not enough to leak water, but if the wind was blowing just right over the roof vent it would kinda back pressure the vents and push a slight smell up through that area.
Its also my experience that it takes SOME time for the smell to dissapate.
Some mentioned dead mouse. My experience is this. *MY* septic has a more "harsh" smell that kinda has a tinge to it in the nose. On the other hand, the dead mouse has a rich deep stink smell, without the "tinge" (i know trying to describe smells, bout the best i can do... hope it helps)