I find Clif bars do pretty well in long term storage, although they are a bit dry after their "best by" date. Lara bars, on the other hand, noticeably degrade and should not be stored long term.

Most canned goods will do quite well stored in a vehicle,unless they show swelling or leakage. My favorite for this application is Dinty Moore beef stew.

Water in clean canteens should be fine for a very long time if kept away from sunlight. I have consumed canteen water stashed in this manner for over a year with no ill effects. I am not sure what you mean by "going bad," but if I saw green algae or something foreign in the water, I would filter through a bandanna or similar and than bring to a boil.

Storing food in a vehicle, in my opinion, means you should have some sort of stove and fuel as well. There are lots of light weight, stable alternatives available.

Nothing lasts forever. Periodically rotate through whatever you stash away. In that way, you can come to your own conclusions about what works best for you and yours.