The true answer is, it depends.
I have one in my Hunter 26 and there are times when you can smell the chemicals or other things. If the pottie is in good condition then the seals can hold a certain amount of pressure. Of course when vented you will smell the chemicals. The bowl needs to be washed at the end of the day or you will also smell stale well you know what. When venting, simply open the seal from the tank and bowl like you are flushing, make sure the lid is closed 'cause it may spray a little. There are some units that allow you to plumb into the boat with vent and pump-out. Done properly these will have less of a chance to be sniffed out. alan