Add at least 5 gallons of water to your RV black tank after dumping. More water when flushing helps break down waste and stops buildup. Using a probiotic tank treatment is also a great idea because it introduces good bacteria that help fight the bad odors and break down waste naturally. This not only helps with smells but also keeps your sensors clear and your valves from getting stuck. Unique makes a good probiotic treatment that works well for this.
tldr: After you dump add 5 gallons of water and add a probiotic tank treatment.