When your boat was built, the industry had -0 experience with sewage holding...it was assumed that any materials suitable for water or fuel
holding would be just fine for sewage and gray water holding. It took about 10 years after the law requiring holding tanks went into effect (1980) for the industry to find out how wrong they all were. By the mid-90s, almost no OEM waste tanks were still metal...almost all went to PE, some to fiberglass
(some of those are only just now finding that SS is even a bad choice for pickup tubes).
Why? Because urine is so corrosive that it typically eats through a weld at a seam or fitting within 2-5 years, and turns the tank--ANY metal tank..aluminum or even 316 SS--into a colander in an average of about 10 years. That does assume that the tank is actually used, btw. Repairing the first leaks
is chasing your tail...'cuz the first leaks
are always only the FIRST leaks...so replacing the tank is the smartest, least expensive, thing to do when you discover the first one...'cuz you WILL have to do it sooner or later...and sooner is cheaper.