One option is to cut a big section out of the top of the tank for access, then cut a hole in the floor of the tank for drainage, then clean it like crazy using the hole for drainage of the detergent solution.
When it is super-clean, weld a wee patch over the hole in the tank floor... watch your fire risk with the radiated heat!!!!... then seal the tank with two part polysulphide.
To leak test the tank, you will have to flood the ship right up to the floorboards in order to see if it is leaking.
Finally, flood the tank to the brim with water (to mitigate the fire risk) and weld a new top onto the tank Iinclude an inspection hatch), and weld the top on again.
In 1997 I had to go through this whole ritual. I never fill the tank to the top ever.... it is simply too hard on it.
To my horror, the tank still leaked if I filled it more than about 35 gallons, so I don't.