[Captain Rich]

"I'm getting a new seal for the floor hatch and taking it off and having a look at the tank myself "

How big is the tank and where is it located? Perhaps you can cut in an opening over the tank and put in an inspection hatch? I have done this over my holding tank and over the fill ports for both fuel tanks. This way you don't have to lift the deck. I would check the vent line first though !