We had that when we bought our FF x210. It had a factory installed sewer vent over the bathroom, similar to this:
We changed it to this 360 Siphon and all was good again. We still had a bathroom smell after someone used the bathroom (with solids).
Our new to us trailer came with this:
No smell. Ever. It's less than $10 more than the 360 siphon and well worth it.