Ronco Plastics Ronco Plastics marine Tanks
(no relation to the VegoMatic Ronco) is your best source for a tank. 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.
You'll have to remove the tank to replace it. Any reason why you can't measure it while it's out? And btw, this presents an excellent opporunity to upgrade the vent to 1" and place fittings where they should be instead of where the boat builder put 'em.