Fruit flies in the black tank

Drain valves are always kept closed except during a dump. I prefer the full flush flow.

If you are going to leave the gray tank drain valve open, make a P-trap in the sewer hose so that there's always water between the sewer and the tank. If the sewer tap is lower than then tank, I put a jack stand under the hose.

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.

Those are sewer flies and also infest bricks and sticks homes. Google it and you will find a multitude of remedies. Here's one on RVs.

Put a small bowl of Apple Cider Vinger where they hang. works good.

They look like fruit fly's but I just called them Sh@@ flys. I would see them from time to time when I would disconnect my hose from the park's septic system.

Not all parks have nicely capped dump pipes. Been to a few that just had a rock over it or even left wide open.

Are you leaving your waste valves open to the septic system? Keep water in the gray P traps and the black tank valve shut with water in the tank. I occasionally get fruit fly's from bananas but not from the septic/sewer.

I had them every now and then when I would be on the road 5-6 months a year. I figured they came up through the sewer line of the gray tank (that I would leave open.) A can of flying insect killer sprayed down the toilet ever so often helped and up the gray pipe when needed.

Then there was the time I was watching TV and heard a scamper under the sink...