I have a '74 Overlander that has the same bath configuration as yours. The black tank also leaked like you describe. I removed the toilet and used a mirror (One of the small ones with the telescoping handle that you can buy at an auto parts store) to look around the inside of the tank. I found a large crack, about 8 or 9 inches long, that ran from the top of the tank and down the side. After reading about black tank repairs on the forum and consulting with someone who had done this kind of repair, I decided to tackle the repair job myself. I used Scotchweld 8500 (search Scotchweld on this forum) to glue the crack back together. It filled the gap and bonded so well that I knew it was going to hold even before I tested it. And so far, no leaks! I am very confident that this repair will hold.
To do this job correctly, it is necessary to remove the tank.
Hope this helps!