Since the tank is metal (aluminum I assume) you really need to remove it and inspect it carefully for corrosion pits. The tank was probably sitting in moisture for some time and that sort of thing can be the death of an aluminum tank.
For the stringers, if you drill a small (3/8") hole on the inside face of the stringer just above the hull surface, that should give you a sense of what is inside and if any water comes out. Do not drill more than what is needed to go through the fiberglass side of the stringer, avoid going into the hull itself. You can drill in several places along the stringer to get a sense of if there is water in some places, but not others.
If all is good, you can seal the stringer back up with an epoxy/fiberglass patch.