Sorry, didn't mean to question your expertise, merely wanted to relate my personal experience. An additive to remove water is ok, AFAIK, since it is essentially an alcohol (isopropanol?) that is highly hygroscopic. However, you woudl have to add it at the end of the storage period, not at the beginning, as it could otherwise have the opposite effect and increase water in fuel. Plus, you'd have to see what effect the alcohol has on the fuel system. It burns ok, but as with E85 it can damage some gaskets etc...
Another consideration is that fuel has improved in recent years, and I believe many companies add stabilizers at the factory. I think these days - condensation aside (and as said above that is not an issue for the Prius II), fuel is fine for at least a few months. Currently, I fill up about every 1-2 months.

had the same experience in Platina, Ca .. took 2 tanks and a bottle of gasline antifreeze.. above ground tanks should have been a giveaway.. but hey.. when ya need to buy gas you have little or no choice sometimes...

never buy premium from a one horse town!

With Guns, Trucks and Beer, Who Needs Women?