[Sans Peur]

If you have an odor, especially after someone flushes the toilet, there are two things to try. First and easiest is go to any boating store and buy some holding tank deodorizer. Pour it into the toilet and flush it. That will cut down the smell considerably.
If you are still getting a smell, change out the vent filter. If the holding tank was ever over filled sewage will get into the filter and after that it is pretty much done. They are not cheap, but pretty easy to change.