Horrible smell from RV toilet - any recommendations?

In our 8th year of full-time and have never used chemicals, just make sure there is some water in tank after dumping and generous water when we flush. We don't flush the tank every time either, just every month or so. All of the items listed above except chemicals need to be checked. The chemical is a deodorizer - covers up the odor, doesn't correct any problem and make it go away. Good luck.