I believe it's generally oxidation that's your problem with most fuels. Stored in an air-tight container and they should last for a long time. Of course, most fuel tanks aren't air-tight, so they will degrade over the months. Diesel less so.
There are plenty of (anecdotal) examples of vehicles being stored for several years and being in a condition to run on their remaining fuel afterwards. Although, they generally work better if mixed with fresh fuel.
Given a car park full of vehicles, a lucky mechanic might be able to scavenge up enough viable fuel to run one of them. Assuming the tires, battery, plugs, etc are all still in a useable condition. Again, a suitable pool to scavenge might solve that problem.
That said, I agree that properly stored vehicles (or fuel) are your best bet. And establishing a means to create fresh fuel is better yet.