this is a possibly bad suggestion, but I'm offering it in the spirit of experimentation.
I hate the smell of diesel, and I'm not the only person who has found that diesel will foul containers and pots and pans used to catch it during one maintenance chore or another. A stainless steel cooking pot will be unusable for a long time after it's been used to hold diesel. I would think that your water tank will be fouled for quite some time, so this suggestion is offered in the hope that the contamination might be mitigated somewhat. The caveat is that I have never tried this. I would be interested in knowing, if you choose to try it, how well it works.
As Brenda suggests, you can probably siphon off some of the diesel through the inspection port. Find the top of the liquid line with a stick inserted into the port (it will be most evident if you tie a paper towel around the stick first. Then mark the siphon or hose that you insert to remove the diesel - so that you take as much diesel, as little water, as possible, from the top of the tank.
Continue drawing from the top of the liquid line (or just below the diesel line) until you're reasonably certain that most of the diesel has been drawn out. Then try an oil zorb fabric inserted into the tank through that inspection port, again keeping it at the top of the liquid line. Since these fabrics do not absorb water, but only diesel or oil, you will have a reasonable chance of removing most of the last of the diesel.
Then use dishwashing liquid to disperse the diesel clinging to the sides of the tank, again removing the liquid from the top of the tank, probably adding water to keep the liquid level from dropping down and thus adding diesel to the previously uncontaminated sides of the tank. When you feel you have pretty much gotten the diesel out, including what would have clung to the sides of the tank, drain the tank, fill with detergent and water, let it sit with the inspection port open. then drain and rinse. If you can still smell diesel (horrible stuff), do it again.
The problem with water tanks is that the interior baffles will provide lots of surface area to be contaminated and make it difficult to get all those nasty drops of diesel out completely, so my strategy is to keep it from coating the sides of the tank and baffles as the water level is drawn down, as it would if you simply drained the tank.