Just a suggestion? Push the pedal on your toilet and get a toilet brush down inside the neck of the toilet. The way they are designed, there's a hallow cavity inside there with all kinds of ledges for stuff to stick to when flushing. Give it a good cleaning. It might be something just that simple.
FYI, a couple times a year I add a cup of bleach and clear water in my black tank and go for a drive. Often when returning home on the last day of a trip.
Then at home, I dump the bleach water and rinse the tank once. I have a septic tank, so can dump there with no issue. I have found doing this occasionally keep the tank from smelling, as the bleach water pulverizes the entire tank when on the road.
There are folks that say their tank never smells and they never do anything except dump it. Evidently, you and I must have over-sensitive noses then.
Ever dump your kitchen sink water on the ground and smell what's coming out of there? Pew-wee! I run bleach water through it also every now and then. Because I can dump on my own ground at home, after dumping at the dump station at the camp ground, I'll add a few gallon of water to it and drive home. Once home, I'll dump it, but sometimes that last little bit will drop on the drive-way ... Pew...wee! That stinks worse than the black tank contents ever did! I end up flushing the drive way down with the garden hose to get the stink dissipated.
