Alright Here's the Deal

I posted earlier about a septic odor all over the house.

Problem solved we had a huge clog in the PVC (4") in the slab. It took Liquid Fire, the largest snake and finally something the plumber called the "sewing machine". No more clog

And for the coup de gras, apparently there's a sagging place in the pipe where the slab has settled. The plumber thinks its under the kitchen just before joining into the mainline out to the septic tank

He said he's not sure whether pipe has broken open and were losing water into the slab (pipes are buried in the concrete). He said look for water around the house and cracking.

Any thoughts from your experiences

drain

I am not sure what a "sewing machine" is or how effective Liquid Fire was, but if the plumber had to use it, then I question whether he even had the correct snake. There is almost NO WAY to diagnose whether there is a sag in the line without using a video camera. Unless the septic tank plugs up so the line can fill with water, and it has to be plugged for days, there is no way water can come up to the surface around the house where you could see it. To do that water would have to rise above its "own level" and gravity, (plus the laws of physics will not allow that to happen). I think he was either trying to "dazzle you with his brilliance", or "baffle you with his BS".

I would think its the latter...
I've never heard of a sewing machine used for drain cleaning either....
I have also never used liquid fire to clean a drain...