Had similar problem. I think the leak was through the threads between the main pump out hose connector and the tank.
In addition I inspected and replaced the small ventilator hose which was kinked at the top near the hull/deck where it exits. I added an L connector to keep it from kinking. Thought maybe tank pressure was building up in summer heat contributing to leak.
Replaced the main pump out hose and sealed the threaded connector fitting to tank with 3M 5200.
The tank itself is very thick-walled so I doubted that it was cracked anywhere at the bottom where I could not see.
Cannot attest to this fix as it was done this season but so far so good...
