Not sure what trailer you have, but in mine, I have three holding tanks. One gray tank for the kitchen, one gray tank for the bathroom shower and sink, and one black tank for the toilet.
The culprit could be one of a few things:
No water in the toilet bowl, possibly allowing smells to make their way into the trailer.
No water in the pea trap of your shower or sink, allowing smells to make their way into the trailer.
Bathroom vent is exhausting more air than what's coming into the trailer, thus pulling air from where it can (toilet, shower, sink, storage area, etc).
What trailer do you have?
What does it smell like?
What's going on when you notice the smell (after you've been towing, stationary, windy outside, exhaust fans on with windows closed, etc)?