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Borescope/endoscope should be part of boat tool kit. $30 on Amazon. You can use it to inspect bottom of tank, side of tank against hull, area around fill pipe, and then stick it in the tank to examine inside seams. (You may have to push some sludge out of the way.)
Non-destructive, will give you a lot of information regarding surface rust (not 100%) and will also help find items dropped into the dark corners of the bilge.
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