Well, first...
I'd check for leaks of course , but unless you find any, the next thing I'd do is run a 1/5" hose from the fill fitting on the tank to a deck fitting (which you'll have to install), so you won't have to drag hoses through theboat any more...that IS a pita! Whether to vent to the outside of the boat or the inside depends on the tank location...if an overflow is only gonna end up in the bilge, you can vent internally, but if it's gonna turn the cabin into a lake, I'd run the vent line to a vent thru-hull. Water isn't sewage...it just needs venting to keep the tank from being overpressurized or pulling a vacuum, so it doesn't matter where the vent goes or what size it is, as long as the tank IS vented...somewhere.Because the system has never been used, there's not much it needs except a good "dusting." I'd just flush it out very thoroughly with lots and lots of clean water. Odds are, the rubber and neoprene parts in your water pump have prob'ly dried out and are toast by now...I wouldn't trust 'em even if the pump does work when you first try it. Whether you want to rebuild it or just replace it depends upon how tight your budget is vs. how much time and effort you want to put into it. Water pumps aren't expensive...valves and diaphragms are in the $10-15 range.Where are you off to that you need to start carrying water?