Gnats in the black tank

We leave both the gray and black valves closed until we're ready to dump, so you are probably right in that they are coming down the vent pipe. I also leave a few gallons of clean water in the tank after we dump to prevent the toilet paper/poo pyramid from forming. I am certain that I have some of the enzyme treatment in the RV, and I may ride over this afternoon and pour that into the black tank. Of course, I'll be armed with a can of Raid when I open the toilet. They don't seem to be bothering the gray tank. Maybe this is just a one time thing because we have had so much rain this season.

I think the sewer flies can get into the tanks via the roof vents.
I had them once in the black tank and sprayed a bunch of insecticide in it. That took care of them.
They can also get into the gray tank drains from the park sewer line if you leave the gray tank valve open. They can come right up through the sink or shower trap.
They breed in the gunk on the drain pipes.

Pouring boiling water down the shower or sink drain will kill them. You may have to do it a couple of times though.

My guess would be sewer flies which breed in most sewer lines. When we leave the dump valve open, they can access the gray tank and may get into the black tank even during dumping. They're right at home in an environment of moist solid waste, just like you'd find in your black holding tank. A spray can of insecticide will provide temporary relief. But the eggs might not be reached. An enzyme-based treatment for the black tank would be my next step. As a last resort, and one I have used once with a severe infestation, is bleach. I filled the black tank with water mixed with 8 oz. of bleach and let it sit for a couple or three hours. After I dumped, I then refilled with just water as a flush and dumped again. Problem solved.

How do I keep them from setting up house in my tanks?

After changing a couple tanks and rebuilding a toilet I don't know you can entirely prevent unwanted insects. I think they fly up or down the vent and lay eggs. I've to leave my dump valves closed until I need to drain them. Helped somewhat. But didn't solved problem 100%. I'm just glad most insects die quickly and hopefully before laying next generations eggs.

That's a good idea, Scott. I'll be ready for them next time. We didn't have any chemicals in the black tank. I wonder if that would help?

I would be ready with a can of bug killer and blow it inside the tank through the toity.

Never happened to us before.