We had a similar problem first time out this year.
Our toilet dumps straight down into the Black Tank a distance that seems line 24 inches. There appears to be some kind of a flange between the two that `hard stuff' can adhere to. Don't know the purpose of the flange.
We sprayed it with bleach but that just covered up the odor with a nice clean bleach odor but it didn't get rid of the stuff. We finally bought a long skinny brush, like a toilet brush, to wipe it away. Problem seems to be solved.
The next problem is what to do with the brush!