It's a 10 gallon Groco tank with integrated macerator pump. While the GW manuals are very detailed and provide you most all the info you need on parts (model #'s, etc.), surprisingly it does not list the size of the head holding tank. I recently spent 6 hours face to face with that tank, halfway inside the stbd bench, bent and twisted like a pretzel, working with one arm most of the time, to replace the macerator pump.