Are you experiencing " unpleased smells sometimes ?
If you do a search you will find volumes on this subject ranging from various additives/treatments through home made filters to what (to me) was the simplest solution on our RT25SC. I installed the Big Orange Filter. It was inexpensive, easy to install on the tank, includes a valve to avoid a tank collapse when pumping out and is easy to refill. No, I DON”T get money or other incentives from them. It was just a great solution to an odorous problem.
Open the lid of the seat/box at the stern and lift out the liner. It lifts easier at the front. You now have access to all the stuff at the stern like the rudder post (that needs a couple of grease shots once a year. You now have the tank in front of you. On the top you will see the access cap that unscrews and you can clean out the tank with a hose when necessary. You will find the point where the feed from the toilet goes to the tank as well as the vent pipe that goes to a fitting on the transom. There is a photo of the Orange filter installation in the “Modifications” album on my photos. Send me an email if you can’t find it. I used the Big orange filter original and bought the refill kit. I believe the pricing on the website is in $CDN so not so expensive for the US buyers. If you put some sort of filter in the vent line and it creates a restriction and forget to remove the access cap before pumping out there are documented cases of the waste tank collapsing. The big orange filter has a relief valve that supposedly prevents this but I try to remove the cap, just in case. Feel free to email with questions.