Boat Odors
I believe your Catalina has a plastic holding tank. Unless there is a leak at a seam the smell is not from the tank. You hose/s may be the problem. Wrap a warm damp towel or rag around the hose for a couple of minutes. When removed if it smells then your hose is bad and need to be replaced.Bilge odors can be as bad if not worse then head odors, investigate all possibilities.The best tank, head and bilge treatment products are from Raritaneng.com, try their KO and CP products.Search the archives on this site. Peggy Hall (Head Mistress) is the expert and her book is available from this site, there are volumes and volumes of information available.I have a 1988 Catalina 34, with her book and following her advice I have no (Zero) head or othersmells on my boat.