Physics, my friend.
The vapor pressure of water is 0.023 atm. So one liter of air capacity in the tank can hold (at most) 0.017ml of (condensed, liquid) water. Of course that's the theoretical maximum AND it assumes both air at 212F to start with and then that it all condenses (neither of which is true in the real world.)
You need the water vapor in the tank to go into supersaturation for it to condense out since diesel fuel (fuel oil) does not absorb water vapor out of the air. This means you need high humidity in the air and then the air in the tank must cool enough to produce condensation. Then you must admit more humid, warm air into the tank in order to get it to happen again. Car/truck tanks are not free to communicate with the atmosphere outside; there is a vacuum/pressure regulator (to prevent the tank from collapsing or exploding) which essentially prohibits exchange except when the cap is off or under temperature changes severe enough to cause the cap to vent either direction. When you fill the tank of course you expel most of the air; what's drawn in as you drive occurs under (mild) vacuum as you consume the fuel.
I ran these calcs years ago for my boat (which had two 300 gallon tanks) for grins and giggles and determined that the expected accumulation in a tank kept half-full over a year's time, WITH an atmospheric vent (because all boat tanks ARE free-vented to the atmosphere) was approximately one once of liquid water in an environment where temperatures ranged from freezing to 95F on a seasonal basis (roughly accurate where I am) in the WORST case. At that point I stopped worrying about condensation as a potential risk but paid a LOT of attention to the possibility of getting a fill where they pumped water into the tank along with the fuel!
(My response to this was to put in a two-stage filter system with the first being a bulk filter that also had a hygroscopic element that would not pass water. Thus, if the tank pickup DID suck up water it would shut the engine down from fuel starvation rather than destroy the injectors. That plus some gauges and a manifold to make isolation and replacement easy would tell you instantly what was going on, and a bypass/priming pump made fairly easy the process of clearing it and re-priming the system, if it happened.)
In short if you have water in your fuel it's coming down the fill pipe, not condensing out of the air. And if it's coming down the fill pipe demulsifying it is exactly the wrong thing to do; the only way to deal with any material amount of water in the fuel without destroying things is to stop it from getting into the IP and physically remove it.
Incidentally if it gets through the IP and into the injectors there is a non-zero risk of it flash-boiling in the injector tip and blowing the tip off. On common-rail engines this will instantly destroy the cylinder involved because the result is that the HP fuel rail is now open to the cylinder; on a unit-injector or IP-pump engine you'll know immediately if it happens and be able to shut it down, but you're still not going to like the damage especially if the tip that is blown off goes somewhere it shouldn't, and it usually does.
