I added a bathroom on to my trailer. It has a commode, shower, and sink. I forgot to put air line up through the roof and now when you take a shower, air is forced up through commode. Backtracking up sewer line from where I hooked in, is the commode, then shower, then sink. If I spliced in the line to put air exhaust, where should it be?

Does the toilet bubble or gurgle? If you don't wish to run a pipe out the roof. You may add a AAV(automatic air vent) to the vanity. Let me know about the toilet. Back to you, Tom

Does the toilet bubble or gurgle? If you don't wish to run a pipe out the roof. You may add a AAV(automatic air vent) to the vanity. Let me know about the toilet. Back to you, Tom
Yes it does gurgle. I was hoping there was an easier way to fix it other than going through the roof.

Yes it does gurgle. I was hoping there was an easier way to fix it other than going through the roof.

As Tom indicated, you can possibly add an AAV under the vanity.