If you remove the knob on the water inlet valve (set screw) you can screw out the "valve" ( 1/4" stainless steel threaded rod with a rubber disc on the end)that will provide you with a golf ball size hole into the tank. With a inspection mirror and a bright pen light you can look around the tank for signs of rot. In the original owners booklet they recommend pulling the boat every two weeks and letting the tank dry out. Bob