I'll bet you always wanted to know why does my boat always have a lingering odor that smells like the head or your neighbors boat does?
Discharge, macerator, and pumpout hoses all have a finite lifespan. Once these deteriorate, gases can leak through them. Deteriorated hoses often are gummy to the touch, indicating excessive age. Replace the hoses with a high-quality waste hose. Also, if you use an aerobic "sweetening" product but don't get enough oxygen into your holding tank, or if you also dump in anaerobic products such as household cleaners, the aerobic product won't work well, if at all.
How can I fix the horrid smell outside of the boat when someone flushes?
Consider adding a charcoal filter into the vent line. Make these yourself with parts available at most hardware stores, then fill them with activated charcoal, which you can find at a pet store. Or purchase one of the commercially available products.