I have an easily removed septic tank cover on the septic tank at home. Because I dump my trailer tanks into my septic tank at home, I remove the lid quite often. This year there seemed to always be something like gnats (or maybe, baby flies) down the pipe and would fly out every time I opened the lid. Never had any in the house, never had any in the camper.
I spend a considerable amount of time and effort flushing and rinsing my black tank in my camper. A lot of folks do not want to, or just won't take the time or effort to do an extra flush, to work a little harder to keep the tank clean and odor free, and build up free. Apart from smells, this is another reason why I work so hard to keep the tanks clean, as evidenced by my septic tank, that just sits there for years and years and has no way to get clean out. The "contents" attracts the bugs. The cleaner the tanks, the less attraction for bugs.