Check our discussion under "plumbing" called "holding tank smells when traveling
". Two of us have now reported the device called a sewer vent check vent missing. I sort of described what to look for in my my post indicated above.
You'll find a picture of the 1?" vent nearly at the bottom of this link page:
Look for a small open plastic pipe stub with inside 1?" threads somewhere near a trap in the vicinity of the smell. Our smell was coming into a kitchen cabinet because of the wall openings under the entertainment center leading to the bathroom (whew) sink... It really was the bathroom sink even though the smell was coming into the kitchen.
If I recall, there are several of these vents located in your unit, all in hidden, out-of-the-way spots.
Good luck, let us know how you make out.