We have double sinks in the bathroom that have backed up off and on for months. I used a thin auger first and it
Went from one sink and came up the other with no gunk on it. I took off the ptraps and they were clean. Then we did the baking soda/vinegar, and gradually went to Drano and a product from Lowe's my negighbor recommended as fail safe. Nothing worked. Then I got on the roof with the water hose and dropped it down the vent until it stopped, and sprayed for an hour off and on while the wife cleaned the gunky water from the sinks into a pail. That did it for a few weeks, now it is back worse than ever, to where any water used in the sinks just sits in the pipes almost to the top. I am assuming it is in the cutoff to the main drain but don't know how to get to that. We have a slab
Foundation.

You have now learned what the rest of us already know. That chemicals only work in TV ads.

I got on the roof with the water hose and dropped it down the vent until it stoppedThat was your first smart move. The blockage's down in under the slab.

don't know how to get to that. You were already there but just didn't push it far enough.
The solution would be to take a 1/2 or 3/8" snake back on the roof and snake the roof vent. Putout enough snake to reach the base and 15 more feet. Good luck, Tom