Hi,
I don't post very often, but this topic is something I am very interested in as well. I have some comments I'd like to add to the thread.
I am considering storing water in underground storage. The reason I'm thinking about doing this is because the earth's temperature below 6-7 feet underground stays at a constant temperature of around 50-70 degrees. And this is all over the earth. I have a few sources for these figures somewhere. I'll post them later. Just want to get the idea out here first.
I know this information doesn't really apply to your water storage needs because you said you wanted to store your water in your car, but maybe you and others can use this information.
I also have a comment on plastic milk jugs. I know that plastic milk jugs may seem flimsy, but I know they are very sturdy. We used one as a childish prank filling it with some chemicals to get it to explode. And let me tell you it took forever for anything to happen. Nothing happened so one of my friends started kicking it. Still nothing. About ten minutes later we were actually throwing this thing into the ground with all of our might. Still nothing. Finally one of my friends gets this thing and throws it with all of his might at a boulder and the thing finally explodes in a shower of bubbles and foam. Lesson: Don't leave teenagers alone to entertain themselves. The other lesson: Milk containers don't break or even leak under extreem abuse. I'd like to point out that it had a twist on cap not a pop on tab.