I had a bad smell in my boat, and if it has a lot of wood inside up against the wall, check the wood and see if it has mould etc. on the back side if at all possible. I ended up over a a number of years replacing the wood panelling that was next to the hull, as well as some vinyl covering with foam backing that had rotten. Now there is no bad smell in the boat. Nigel Calder recommends putting some vinegar in the heads and flushing it through once a month. This helps in minimizing the calcium buildup and it kills the odours as well.