That's not a great location for a waste holding tank...boat builders have an unfortunate habit of putting 'em where it's easiest and then building out the boat around 'em. If you can thoroughly clean it out and plug the fittings, we can look for a better place to put the new one.
Ronco Plastics Marine Tanks - Ronco Plastics
(no relation to the VegoMatic Ronco) Ronco Plastics marine Tanks
is your best source for a tank. They make TOP quality thick-walled water and waste tanks for a very reasonable price and has more than 400 shapes and sizes, over 100 of which are non-rectangular (which means you might be able to fit a larger one in the same space), and they install fittings in the sizes and locations specified by the customer when they make the tank. So you'll have a lot of size/shape options...one of which will aomost certainly be right for a good location.
"If you can't explain it to a six year old, you don't completely understand it yourself."