V-Berth is the vest location
It's within 6' of the toilet and will allow you to run the vent line forward at an angle that'll keep the tank aerobic, which is the key to odor elimination.Ronco Plastics http://www.ronco-plastics.com is your best source for a tank. They make TOP quality rotomolded PE tanks for a very reasonable price and have more than 400 shapes and sizes to choose from, including 100+ that are non-rectangular. And they install fittings in the sizes and locations specified by the customer when they make the tank. You should find the link below useful in specing out and installing your system.As for whether to keep the y-valve...If you spend most of your time at sea beyond the "3 mile limit" where you can legally flush directly overboard, I'd keep it. If you spend most of your time at the dock or in the bay or other close-in coastal waters, you might as well take it out.