[Jimmy]

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.

Make sure and check your vent and make sure its open , otherwise you risk clasping your holding tank , if its not already cracked .
I have just did mine , had a  smell, vent was plugged (had to replace it) along with new vent hoses and filter .
also had to change valves as they were frozen and wouldn't move as I had 2 holding tanks . Now its smelling nice with no odor .

Captain Jimmy