There are a few reasons why your RV might smell when dumping the black tank.
First, the tank might not be properly ventilated. This can cause odors to build up and eventually escape through the drain vents. Additionally, the tank might be full and in need of emptying.
Finally, there might be a clog in the drain that’s causing sewer gases to build up. If you think there’s a clog, it’s important to clear it as soon as possible.