Well, when I refinished my hull I found a spot where some PO had covered a 1/2" hole in the hull (at the waterline, no less!) with some gooey substance that I was able to push in with a finger
The point is, unless you've really verified it, there's a chance that the hull itself isn't watertight. Until I did the work, I did not find this, it had been nicely painted over. I think that the caulk or whatever it was, was still holding the water out, but it was only a matter of time.
With a hair dryer (set on cold) you should be able to test whether the tank is sealed. Work some adapter between dryer and inspection port from a sheet of plastic and duct tape. Turn on. Check all surfaces with soapy water (inside and out).
(Just in case the hair dryer is able to create so much pressure that it exceeds what the tank can hold, put a flap in the adapter as a bypass. That will relieve any overpressure and allow some air to stream past the dryer motor for cooling. You don't want to inadvertently blow apart a seam in the tank or have to explain to your better half why the hair dryer died)