Replace Hoses
I had the same problem on a 1988 Catalina 34. I replaced all the head hoses with the newer hoses. I bought the white, expensive stuff. Head odors supposedly won't leach through in later years. This cured my problem with head odors in the boat, haven't even considered changing the holding tank. Try the easier stuff first, I always say. Oh yeah, and to help prevent problems, use Odorlos. If you use it and follow their directions, you will be amazed at how well the stuff works. It will take about three or four pump outs after you start using it before you really notice the difference, but I guarantee you that you will notice that difference!