Replace the tank...urine is so corrosive that it will turn any metal tank--even stainless steel
into a colander in an average of about 10 years. It typically only takes about 5 years for the first leak to occur at a weld in a seam or a fitting...and the first leak is always only the FIRST leak. When you get it out, you'll see that the bottom is covered with pits that are about to turn into holes. Seamless rotomolded polyethylene is the recommended material for waste tanks
If you're in the US, your best source for a tank is Ronco Plastics Ronco Plastics Marine Catalog
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 (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. Turnaround time is about two weeks including time in transit.
And btw...welcome aboard cruisersforum.com!