The v-berth is my favorite location for a holding tank on a sailboat, the distance from the toilet is fine and there's an excellent chance that Ronco Plastics will have what you want. They make TOP quality thick-walled water and waste tanks for a very reasonable price and have more than 400 shapes and sizes, 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. There are retailers who sell Ronco tanks, but Ronco sells direct for a much lower price...and they're great to work with. Their current catalog is here
Ronco Plastics marine Tanks So many people told 'em their previous catalog is much easier to search that it's still available too
Ronco Plastics old marine catalog
A word about their tank drawings....there is no top or bottom until the fittings go in, and you'll decide where they go. So rotate the drawings, flip 'em end for end...I'll bet $1 that you can find a good fit in the size you want.
A Plan B if your budget can stand the price: There's only one small NDZ on the whole Chesapeake Bay--Herring Bay...the discharge of TREATED waste from a USCG certified Type I MSD is legal everywhere else on the Bay, even Annapolis harbor. Check out the Raritan PuraSan
Raritan PuraSan Promo Sheet It wouldn't replace a tank, it would be in addition to a tank that could even be smaller than your existing tank. I suspect you'll have a bunch of questions about it, so If you'd like to discuss in more detail than is practical in a forum thread, send me a PM.
--Peggie
