Arthur gave you good advice...
Your description of the knob on your toilet that switches from "flush" to "dry" tells me your toilet is either a Raritan PH II--the best manual toilet in its class--or its predecessor, the PH II (if the knob is metal it's a PH...if it's plastic, it's a PH II). You can get the manual for the PH II here: http://www.raritaneng.com/products/toilets/manual/ph_2.htmlAnd this should be helpful to you too: http://www.raritaneng.com/information/faq_topics/faq_ph_II.html#a6After sitting unused for 3 years, and prob'ly no service for years before that, I strongly recommend you "rebuild" it (put a service kit--all the rubber parts, valves and gaskets--in it)....contact Raritan to get the right kit for it.But first you need to flush all the antifreeze out of the system. That's fairly easy: open the intake seacock--that'll be the one that's connected to the smaller line going into front of the pump (see the owners manual)--and pump a LOT of water through the toilet in the "wet" mode. Then you'll want to flush it all out of the tank.The Whale Gusher dumps your tank at sea through another thru-hull. You've only found one other one...if it's the same size as your head intake, it's your sink drain. If it's 1.5" ID (inner diameter) and connected to the pump--which is connected to hose coming out of your holding tank at the bottom--it's the one that dumps your tank. That's only legal at sea at least 3 miles from the nearest point on the whole US coastlineIn short, you need to explore your plumbing and figure out where it all goes. And check all the hose connections while you're doing it...'cuz you don't want a hose to part company with an open thru-hull. As for how to empty your tank in a harbor...there should a deck fitting marked "waste" that looks like your fuel and water fills. There MAY be a y-valve in the tank discharge line, or it may be only a tee or wye fitting. To pump out the tank, connect the deck pumpout to deck fitting. (Get someone in your marina to show you what to do). You'll find the link below useful in maintaining your toilet and tank properly. And I'll be glad to answer your questions.