Someone tipped me about this experience which is highly relevant for this thread:
http://zombiehunters.org/forum/viewtopic.php?f=32&t=73649 (no, I'm not a member there).
It is the experiences of a very well prepared couple that got stuck on a highway. They planned to - and were well equipped to - spend as long as necessary in their vehicle, but were ordered to evac by the police, who transported them to a truckstop or whatever. (Whose heating system didn't work, by the way...)
Their story set me thinking: Items dedicated to making indoors sleeping a bit more comfortable may be worth considering. I think most of the people stuck in their vehicles would have preferred to stay in a hotel if available.
Another scenario is that the police or other authorities order you to leave your vehicle, which may or may not be to your liking. (Discussing wether this is within their legal boundaries to do so is another topic - but those cops have a) no reason to believe whatever you say about "being prepared" -after all, most people aren't and what makes you think the cop should consider you as the sole outstanding exception?- and b) if they leave you and something bad happens they will have a very big problem.) Like it or not, legal or not - it will be VERY hard to say "I'm not going anywhere" to a cop that insists on evacuating you when you're stuck in a blizzard.
In either case, having some toiletries such as a toothbrush, a towel and clean underwear will make you stop feeling like a clumsy polar bear at a cocktail party. In my experience, it is no problem roughing it for as long as it takes when I'm in the bush - but I feel highly uncomfortable not being able to clean myself when I'm in a more urban and civilized setting such as a hotel. Not really related to survival, but highly relevant for your comfort and hygiene.