welcome to sailnet, slrrls2000. what you had in the v birth was an old marine head. small pipe for water inlet, large pipe to jettison or flush , well you know what. the hose leading from the twenty gallon fresh water tank, went to the sink. the drain pipe was for said sink. so you win a smiley face for your correct assumptions. good luck with your project, and post pics of your progress, please.........
Self Proclaimed Genius