The above boat odor book by Peggy Hall is the bible. It will contradict some of the advice you received above, so I highly recommend getting it.
It may be really good news, if your head was not used, because the vast majority of boat owners improperly flush their heads. That causes the waste hoses to permeate and you can't fix that, you have to replace the hose. You can test your hoses by wrapping them with a hand cloth soaked in hot water. Remove it, after it cools, and take a whiff.
Your head could probably use a rebuild kit, which aren't terribly expensive and necessary every few years anyway. Buy two. The joker valve is probably dried out and a new one would be in that kit.
Good luck. Read that book!