I can hear water leaking when I fill or partly fill, so I need to have access to this tank to check where the leak is coming from
Are you sure it's the tank?
It could be the fill hose which is failing. My boat is a hair older than yours and I had to deal with the two tanks, water, filler hoses last summer. One actually split, one was showing cracks which would have sooner or later. I think the worst had been in trouble for several years before.
Try to get a good hard look at the filler hoses. Maybe that's where the water is coming from which would also be in keeping with you hearing water running while filling. It could even be rotted off clamps releasing the hose from the tank nipple.
Mirrors, flashlight can often help. Tie them to a stick if needed.
Can you access the fillers from the engine room vents by removing them or through lockers.