I must say, smell is not a significant issue on my boat.
I bought my boat from a Charter Company, and as often usual, the septic tank was disconnected. It's a big black thing in the cabinet in the bath room, which I believe was originally meant to hold bad weather gear; it can drain to the bilge.
The output of the toilette goes straight to sea, which in my case is not a real violence of any rules. First here in Slovenia and Croatia, septic tanks are not a legal must, and on the other hand, we NEVER use our toilette in the Marina. We have good bath houses and toilets, no need to pee or worse in the ship's head.
I have not changed the situation, in fact last autumn I took out the tank and will rebuild the cabinet as storage for cleaning stuff, etc. with shelves. The suction opening on deck was leaking too, closed that up and sealed it.
As mentioned in the Marina or near shore we Never Ever use the boat's toilette. Now people may point their fingers at me and blame me as polluter or whatever, I don't feel like that, and I'm not breaking any law (as far as I know).