If you have the vents open and flush the toilet... bad odors come UP from the tank into the RV. I turn the ceiling vent (in the bathroom) off before I flush. (and when I'm dumping the tanks I make sure that all the ceiling vents are not running.)
I have used a couple of cups of pine sol on very hot stretches. Pour a good amount into the tank through the toilet. I don't use it on a regular basis but for those couple of hot weeks in the summer it sure seemed to ease the odor problem.