I had a similar problem on my '79 30 Cherubini. I read all the posts in the archive and weighed the options. Finding leak and fixing from inside the tank required cutting the old tank (baffles in tank) in numereous places on the top to gain access. Assuming you could find the leak point(s), labor, material costs and longevity of fix seemed to be problematic. Opening the tank to install soft tank required as much labor and cutting as the following:
Cut top off settee (or wherever your tank is located), call Dave a SBO and order a replacement tank that fits the space (maybe a bit larger than original), as they come in a myriad of sizes and configurations. Install the new tank with appropriate fittings and tie downs. You can do these steps on your own. Then, you could have a pro re-install the settee top so that it will be structurally sound, but re-removable if ever necessary.