Black flies in toilet

Every time we park our RV unused for any period of time we get zillions of black flies coming up out of the toilet. I scrub the well of the toilet bowl with a brush as far down as possible but obviously I'm not getting the job done. We always drain the black water and treat with chemicals before storage. Does anyone have a suggestion on how to control these flies? Would something like a mosquito dunk work?
Black flies in toilet

Hey sydneylanier,
Had the same problem. The only way we could solve it was to squirt (don't use the aerosol type) a VERY ample amount of store bought insecticide (household flying insect type) into a small amount of water in the bottom of the bowl and then quickly flush. I would say we used between 1/8 to 1/4 cup of the insecticide and after sitting for about 1/2 an hour the flies were gone.
Hope this helps.
Keep Wavin'