how to clean diesel residue from plastic water tank

I don't know of a way. Perhaps soapy water, but pretty sure the diesel gets into the plastic and is there to stay. Provided the tank comes out easily, those plastic tanks are pretty cheap.

J is right. Replacing the tank is probably the least expensive and least frustrating solution. The plastic used to make those tanks is the same kind as used in oily water separators because it is oleophilic, it loves oil, it attracts and holds oil and is not in any hurry to release it again.

If repeated cleaning with a solution of very strong detergent and hot water doesn't work, the solution to your problem is most likely one of those best of a bunch of bad choices things. Bite the bullet and replace it if it is accessible without disassembling the boat.

X3