Hello texasAUtiger
Bare with me there are several points. If you are parked and hooked up to sewer there are two ways to keep the smell at bay. If you keep your sewer valve open and not allow the waste to stay in the tank the trick is to flush and then fill the bowl with about 2 inches of water this makes a seal so the smell stays in the tank. If you do it this way fill your tank and then dump into the sewer before you leave. That way you can rinse the solids from your tank and save on chemicals.
If you are hooked up to sewer and keep your tank valve closed and wait to dump you will need to use chemicals. Still leave the tow inches of water in your bowl in any case.