It depends.
It depends where your through hull is. Most 27's came with a holding tank at bow underneath the V-Berth. If you lift the cover on the starboard side you should see two valves; the smaller is the intake and the larger is the outflow which could be connected to a manual pump. The outflow is only opened when the pump is being operated. Discharge laws are very strict with stiff penalties so I suggest you familiarize with and follow them. The head should have a lever for wet or dry flush which means that when the intake valve is open you can bring water to help with the flush if you choose. Your head sits under the boat's waterline so if you leave the intake valve open and the lever in wet flush the water will overflow in the bowl and may sink the boat. To prevent this some boats have vented loops above the waterline but most of them did not. If in doubt keep the intake closed and open only when in use and make sure the lever is moved to dry flush after use. Needless to say when the boat is left unatended the intake valve should be closed. We have a sign above ours that reads "Do not put anything in here that you have not eaten first" and that is religion. Good luck.
