Insect prevention in black tank

My wife and I are currently living in my Keystone Outback in central Florida awaiting our house to be completed. We are living in it full time. I travel for work, so when I am gone I leave the front (shower) and rear kitchen) grey water tanks valves open. the black water tank is closed, as she will not even hit 2/3 of a tank in the week. I dump and flush them on the weekend I am at home. We use orange black tank enzyme packs.
My wife is advising there are gnats that live in the black tank, and sometimes fly out of the tube going to the black tank when she flushes the toilet. She does hold the pedal down for a bit to ensure enough water to keep the poop pyramid problem from happening.
I have not seen the bugs yet, but am wondering if she could add some sort of insecticide to kill any insect life in the black tank: but not turn the tank into a bomb or damage it.
The upper ball seal in our Domatec 310 is leaking a little and the bowl will dry out over night. I plan on replacing the seal this weekend.
I do have a "dip" in my dump hose that holds water to make a P trap for the shower grey tank and kitchen to keep sewer gas at bay from the park sewer line.
Has anybody addressed this issue? Thanks for any assistance.