Put ALL the fitting on top if you can, Ed...
You'll need at least 5" clearance above the tank to do that, but if you can, it's the best way to go, because it eliminates standing sewage--which leads to permeation--in any hose. It also eliminates spills when hoses inevitably have to replaced or in the event, God forbid, a connection failure, and prevents waste from running back toward the toilet when the boat is heeled unless the tank is full to overflowing...and, it increases the useable capacity of the tank. Just make sure the discharge fitting is over the lowest point on the tank when the tank is installed. Whatever you do, do NOT put any fittings ON the bottom of the tank...the hoses coming off 'em will always be full of waste with the full weight of the amount in tank bearing down on the hose and connections. And anything a guest might flush into the tank to clog the discharge hose will always settle to the lowest point, which will be in any hose at the bottom...with no way to disconnect the hose to unclog it without a major spill. If you don't have enough clearance above the tank to put all the fittings on the top, put the discharge fitting on a vertical surface AT the bottom, never on the bottom.The tank vent needs only 2-3" clearance, and it should always go on the top...Put it toward the centerline of the boat, so the vent hose has to run across the tank to the hull (or on the centerline at the forward end if the tank is centered in the boat)...that prevents sewage from running our the vent when you're heeled.And don't forget to include a 3" inspection port. E-mail me if you need to discuss specifics in detail.