Just spray some bug killer down toilet with water off.then close toilet blade and let sit. Then flush
Also spray some down vent lines from roof.
Don't leave black tank drain open so the bugs can't get in plus other problems with leaving it open (poop pyramid)
The loop in sewer hose is OK BUT why leave ANY drain valves open so bug can get from sewer system back up into your grey tank.
Sewer flies are common in sewer/septic systems even treatment plants.....don't invite them in. Leave drain valves closed until dump time. They fly but aren't very good swimmers especially upstream.