your tank may not have a pump out pipe, but if it has you should have a deck fitting like the ones for fuel and fresh water (don't mix them up). and also a breather pipe to an outlet usually just under the toe rail height on the side near the holding tank. 2 possibility's for the pipe are 1st, a pipe fitted to the bottom of the tank on the opposite side to the one with the seacock in, or may be fitted to a 3 way joint just before the seacock.
there are plenty of videos on the net to show how to use a pump out station, that would make more sense than me explaining it.
hope you find the fitting
phil