I'll be glad to help you spec out a system
For starters, in your waters removing the overboard discharge thru-hull may or may not be a good idea...it depends on whether you spend most of your time in Narragansett Bay or offshore at least 3 miles off the coast.
Connecting any toilet designed to use sea water to the fresh water system is a BIG no-no...it cannot be done without risk of contaminating the potable water supply, damage to the toilet or both, and every toilet mfr specifically warns against it in their installation instructions. So unless you upgrade to an electric toilet designed to use pressurized flush water (which IMO doesn't make sense on a 27' boat), you can't do it.
However there are a couple of ways to eliminate the head intake thru-hull and supply the toilet with fresh water. The simplest: tee the head intake line into the head sink drain line. Use sea water to flush instead of wasting fresh water...but last thing before closing up the boat, after you've closed all the seacocks, fill the sink with the clean fresh wate...flush the toilet. Because the seacock is closed, the toilet will pull the water out of the sink, rinsing the sea water out of the WHOLE system so it can't stagnate and stink. Next best solution: buy a small (2-3 gal) UNvented bladder...stuff it into a location that's convenient to both the toilet and the head sink drain line. Use a y-valve to connect the fill to the sink drain line, connect the head intake line to the bladder. No other plumbing needed. To fill the bladder, turn the y-valve, run water down the sink.
You're gonna need a toilet...and all marine toilets are NOT created equal. The most durable reliable manual toilet in its class is the Raritan PH II...or the PHC, which is the PH II pump mounted on a compact base and with a shorter pump handle to fit in a tight space. The PH II and its predecessor the PH have been consistantly rated the best manual toilet under $500 for decades. Check it out here
Raritan PH II and PHE II and order it from the sbo.com online chandlery.
You're also gonna need a tank...your best source is Ronco Plastics
http://www.ronco-plastics.net (no relation to the VegoMatic Ronco). They make TOP quality rotomolded tanks for a very reasonable price and have more than 400 shapes/sizes to choose from, over 100 of which are non-rectangular...and they install fittings in the sizes and locations specified by the customer when they make the tank.
Then there are hoses, where to put the tank, where to spec the fittings, how to vent the tank and whole bunch more we prob'ly oughta talk about before you start this project. So if you'd like to send me a private message I'll give you my email address.
Meanwhile, you might want to check out the link in my signature...it goes to a comprehensive "marine toilets and holding tanks 101" manual that help you learn how to install, operate and maintain your new system to PREVENT 99% of problems..'cuz prevention is a whole lot easier, cheaper and less aggravating than fiixing 'em later.
