I had the sewer flies for a year. As you did, I
posted here and tried all the suggestions. None of them worked.
I went to Majors RV in Bourne, MA. I was told to clean the tank with a toilet wand and to use a tank deodorant/cleaner they recommended. It worked.
Unfortunately, I am 1,600 miles away from our RV, and can't remember the name of the product they recommended. It came in different size containers from small to a gallon and was very blue. The gallon was about $32 at the time. I continue to use the product.
Majors RV Repair has a "stump our tech" section. Send them an email and ask what it is.
I hope this helps.