I can tell you this, waste lines will start to smell after a while, the odor from the waste will permeate through the hoses. Visually inspect your hoses, if they look clean, take a clean rag and wipe the outside of the hose. Does the rag smell? If it does, hate to tell you, but you have to replace the hoses. I had this problem on my last boat when I bought it (prior owner probably didn't keep it clean enough) and I did everything possible before starting the project of taking the hoses out. Nothing worked. The trick is to get the hoses as empty as possible before removal. This isn't always possible. So what I did - pump out your tank first! If you can fill it a few times with clean water and pump it out a few times, it will cut the smell down. Then get ready for the real work - bring a lot of rags and a lot of plastic bags. When you cut a length of hose, take a couple of rags, put them in the plastic bag, and jam the end of the hose with it. Press it a good length in. Now put a plastic bag over the end of the hose and duct tape the thing to death and fish it through. The rags in the hose will clog the end and the plastic bags wont absorb or leak the waste out. If its going to be hard to put the new hose back, attach a nylon rope strong to the end of the hose *before* you pull it through it (ask me how I know this), this way you can have a buddy pull the rope later guiding the hose back while you fish it through. It may take extra time and effort up front, but I pulled all my hoses out this way (head=>tank, tank=>pump-out, tank=>macerator, tank=>vent) this way and not a drop leaked. If you have a nice boat, and want to keep it for a while, invest in really good waste line hoses, the price is worth it When putting it all back, try not to create any "sags" in the hoses, otherwise the waste will collect there and in a few years you'll have the same problem again. Its best to do this on a sunny day (believe it or not) so you can let the new hose out in the sunlight for a few hours to make it flexible, its a bitch trying to bend it when its cold. Dont forget to double clamp all the ends. If the ends are tight when trying to put back on the fitting, heat them up with a heat gun or torch (be careful not to burn or heat it to much). Run the lines long before you cut them, cause if you cut them to short, you'll gonna have to run new lines (dont splice). I also took the entire tank out when I did this. And flushed, washed, bleached the heck out of it, before I put it back. This way, the entire system was empty and clean. When I was done, it didn't have a smell at all and it passed my wife's nose test. Good luck. Bring plenty of beer and a buddy to help you with the lines and to drink the beer.