I have an issue where my bathtub wouldn’t drain and it turned out to be the air admittance valve for the kitchen sink. It is the only AAV in our small trailer and affected the tub since the sink drain line joins the bathtub drain line before proceeding to the gray tank. The AAV was hard to find too. It was located in an inaccessible area between the microwave compartment and the under the sink compartment. I discovered it by using a $20 endoscope camera hooked up to my laptop and shoving it up into that space from the cupboard below. I had to remove the microwave and knock out a cabinet wall to get to the AAV. I reached up in there and when I unscrewed the AAV I heard a whooshing sound as the bathtub suddenly drained! I will add that the drain line from the bathtub to the gray tank is quite long and has 4 90 degree elbows and at one point it runs uphill before exiting. It is so tight under there that instead of a p trap there is a special pinch valve trap. I might add that at the time my trailer wasn’t sitting level and was tilted away from the gray tank connection but did not matter.