Now, let's think about this. You have a fresh water system, therefore you have a water tank, fill, and vent. Now, with a holding tank, you'll have a "fill" and a vent. You have one of two options. Holding tank with dockside pumpout, or holding tank with dockside pumpout and overboard discharge. That means a macerator pump thrown in the mix.
My guess is there is a waste "fill" on the deck somewhere. "fill" is an oxymoron when it comes to waste. What, are you going to put sh%t INTO the boat? I always get a chuckle over that. Describe your head system to us. Electric head w/holding tank? Vacu-flush, fed by fresh water? Take pics, and upload via Photobucket or picsaweb. We'll figure this out.