If you have a black tank flush built into your trailer, use that. If not, you can buy a wand (or make your own) and stick it down through your toilet into your tank.
Something like this:
https://www.amazon.com/Camco-Flexibl.../dp/B0006IX7YC
Also get a clear sewer adapter so you can see what's coming out of your tank when you empty it. You'll be amazed at how much "stuff" comes out of your tank after you've emptied it and then run the flusher.
We had the same problem where we couldn't figure out why our toilet stunk so bad. Then I ran the black tank flush and the odors went away.