Gnats

I would plug the vent on the roof and then spray half a can of bug killer into the toilet opening. Let it sit for a while and then use black tank chemicals to help keep them from coming back.
Just an idea, YMMV!

Boiling water
A spray can of an insecticide containing pyrethrins (natural insecticides produced by certain species of the chrysanthemum plant) or resmethrin.

That will kill off the adults.

But you may have to remove toilet and treat the underside of it along with the toilet to tank dump line and the vent line.

The sewer flies thrive on waste. Adults emerge from the pupae every 20 to 40 hours and leave 2 weeks continuously laying eggs which hatch in 38-40 hrs.....on and on and on.

Clean the black tank aggressively.

You aren't leaving black tank drain valve open....correct????

We had this problem at our last park hosting position. The gnats were hatching in a holding tank at the park and coming up the sewer hose when dumping. We poured a couple of gallons of bleach into the holding tank and 1 gallon into ours after we dumped. We left it in there for a few days then dumped again. You may have to treat it more than once.