your air admittance valve is probably either loose, leaking, or both.
I discovered one of mine leaked and they both were loose.
Seems to be a common issue.
Cheap and easy fix/replacement.
Also, yes. I keep a bottle of the blue thetford stuff. Don't pour it in always, but keep it handy for those times..... it works fast to take care of it
well,I do put it in the black most of the time even though it's not always necessary. The trick is generally to use plenty of water and then treatment isn't a must-do.
Anyway, check your air admittance valves