I am having this strange issue with my new to me camper. Its a 2020 R pod 171 so fairly new. I have moved into it full time so I am working thru all the kinks the previous owner never acknowledged. 

The current issue is the wet bath shower drain will only drain when the gray water tank valve is open to sewer. If its open I could shower for forever. If the gray tank drain valve is closed it backs up in a few minutes.

Upon further investigation the kitchen sink is on its own piping to the gray water tank and it can drain fine with the gray water drain closed. This means the sink is not having issues with venting or a vacuum lock. Now for the shower I am trying to locate how the shower drain gets vented to the atmosphere to allow for draining when drain system is closed. I climbed on the roof and checked for a roof vent but only found one for the black water tank. It was also free of blockage.

Does anyone know how the shower drain is vented to allow for draining? I need to find the vent and clear it of debris or something. Thank you!