I plan to leverage my Chemical Engineering income and real estate income to grow my multifamily real estate business significantly.

A word of caution muratic acid is a dangerous chemical that one should know how to use before attempting. 

I searched on the internet for drain flies and tried almost every chemical that claimed to kill them.

Some instances will only require spot treatments while large infestations in non-accessible areas will require a tenting job of the entire home.Doing it yourself - I do not recommend this, not due to the argument of your time is more valuable, but more importantly, apply chemicals without a license to do so could get you into trouble.

If they are not hooking into your sewer system then that is moot.RV's have many other types of chemicals in the affluent that is not present in regular SF discharge..

There's a lot of heavy equipment involved, expensive seeds, and a lot of nasty chemicals.  

I had two business and and a high volume of customers and chemical use.

We have used the chemical treatment and snakes with cutters to keep on top of it. 

I snaked it, used chemicals, plunger, everything.  

Get the chemicals in and balanced, clean the tad poles out of the pool pump screen often, and skim them.