I live in a mobile home. I have a double sink with disposal on the left side.
The water will not go down the regular side of the sink. It is backing up into the disposal. I used a plunger and the water transfers between both side. I removed and snaked the line for about 15 feet. Half filled the sink and the water backed up into the disposal side.
What next ?

I used a plunger and the water transfers between both side. your plunger acted like a pump moving the water from one side of the sink to the other.

I removed and snaked the line for about 15 feet. if you removed the trap and snake from there with no results. Then snake from the kitchen roof vent.
Let me know how you make out. Good luck, Tom