If legal to use in your area, consider installing a Type I sanitation system. I have an older boat that was built with a manual head that flushed overboard. Legal in 1955, but not anymore.
It can be difficult to find room for a tank system on these older boats. You could buy a portapottie, too. On the other hand, the Type I systems like LectraScan and Purasan could be just the thing you need. The actual units take up little more space than a group 27 battery box. All you need is a head, raw or pressurized water and a discharge through-hull. If your boat came with a head, you must have that.
The LetraScan and Purasan are great. They don't stink and you won't be making any trips to the pumpout stations. For its low power draw and economy, I chose Purasan. Going on year three with it and very happy with the system!