I have a seasonal park model and I have a sewer gas smell under the bathroom sink. As long as the cabinet door is closed you don't smell it. This park model is a 2-story so there is no venting through the roof for the bathroom sink. Under the sink there is a gas vent pipe that connects to the piping under the sink. I already replaced the gas vent cap, poured bleach down the sink drain, and tried some Liquid Plumber down the drain overnight but had no good results. Any other suggestions you could provide would be great.

Thanks,
PB

Is there a P-trap? If not one should be installed, if there is a little water should be kept in it. This is to keep the ordors from coming up through the drain.

I am attaching a picture of p trap