My tank is in the stbd laxarette...
...in the cockpit, on the inboard forward corner. Mine is an '82, has a (I think) 15 gal tank. Might be a previous owner's mod? I siphoned most of what was in there into a bucket (a little at a time) and dumped it, then got the rest out by pumping thru the sink, cycled 10 gal of very diluted bleach water thru, then emptied again & refilled. Still has some sediment in there, we use it for washing dishes etc, not drinking. We haul gallons of distilled water for that. Our boat is new to us too, we will probably try to fix this in the winter...