Odour underneath bathroom sink.

[newfiecamper]

Hi. The other morning as I was using the sink in my 2012 salemhemisphere 26rls the sink mysteriously started to fill up and then just drained rather fast. Ever since we have been getting I pissy like smell underneath the bathroom sink vanity. Im not sure if its gray water smell or sewer but im assuming gray. I also assumed it was one of those mechanical vents next to the trap so I replaced it with a new one and seems the smell is still there.would I notice the smell gone right after replacing the ventmatic? There is no smell coming up from the toilet or around it. I can't see or find any leaks but I haven't removed the underbelly yet. I went on top and tied on a wrench and dropped it down the vents to make sure they weren't plugged and didn't seem to be. I am 5hrs from the dealer and would like to find the issue on my own first. I have been stationary at serviced park for well over a month now. All traps are full and I don't have washer hookup. I kinda baffled and like I said it seems to be a venting issue but I don't know where else to look. Anyone got any suggestions??