IMO Why use a good fresh water tank with the studs for a level gauge already installed? A good greywater tank on fleabay goes for $100 shipped. And that tank you have is worth more then that! But i also understand free and having one laying around...
Your vent for the fresh water most times will be with the fill door. Turn it over little fitting vent, big one fill. So when the tank is full water comes shooting out of the hole to remind you to remove the hose...
Grey tanks can become pretty smelly so a P trap is needed!
Put it in a shopping cart and drive it into home depot and starting screwing stuff into the tank to see what fits
That will really piss off the know it all in the plumbing dept
http://www.rvsurplussalvage.com/cata...ategory_id=182 Look here for stuff...