Haven't done it, but...
Hi Ramona! I haven't made that modification, but if I were to hold the grey water aboard, I think I would put the tank inside the cockpit locker, as far forward as possible, and centered. This is where I was going to put a large fresh water tank on my 23 (before we decided to get a larger boat instead ;o). This seems to be a good location, as it is lower than the sink drain under most conditions, and somewhat easily accessible. How were you planning on draining the grey water tank? In this location, you'd need some kind of pump, or a deck fitting similar to a waste pumpout setup. Either that, or your grey water tank needs to be removable...--Jon BastienH23 '2 Sheets to the Wind'H25 'Adagio'