Ours is like Lyra describes, plus there's a Y valve mounted on the sink bulkhead next to the toilet, labeled "Sea" and "Tank", which diverts the toilet output either directly to a seacock or into the top of the holding tank. The whole setup works very well including the gravity draining at sea by opening the tank outlet seacock - we then usually pump a bit of seawater into the tank via the toilet before we close it again.
The pump-out fitting is on the side deck, directly aft of the holding tank, which is behind the shower wall.