A/C smell

I drive a 2019 Freightliner Cascadia it only has 234,000 miles on it.
I noticed today when I was on my way home,
I just pulled into the drive at the yard and I noticed this really foul smell coming through my air vents it's making me nauseated to my stomach and it's giving me a really bad headache.
I also noticed the smell when I turn the truck off and the Apu is working all by itself, pushing air through those same air vents and I can still smell this crazy smell. whatever it is, its really stinks. I've tried everything I've opened up the windows I've tried to air it out.
I've changed all of my air filters, under the hood by the dash, flat skinny paper filter,
And the 2 rectangle air filters inside the cab on the passenger side and under the closet,
Also the apu air filter too.
And this smell almost smells like old stagnant air.. its pretty rough.
After a while you begin tasting that same smell like in your mouth.. I have to go outside and get some air.
This is something new. I've never encountered anything like this b4...
Has anyone experienced this kind of smell??
A friend suggested that maybe the burner in the exhaust regen device quits working, and the exhaust bubbles back into the exhaust, then escapes back into the air vents..
The smell makes sense if it's that..cuz when you pump it from the fuel island it can gag you if the wind blows the wrong direction and you get a strong wiff of it..
The mechanics are all gone for the day.. just trying to see about some idea what it could be.