I second the poncho suggestion - never know when you need to get out and walk - might also include some sort of bag to carry supplies in if you need to abandon the car?
Work gloves? Always useful if you need to do road clearing
I might prefer to have a bit more water on hand
Meds - opinions vary but my experience is that nothing makes life more miserable than the runs - some sort of anti-spasmodic makes life much more bearable
Since you have the space I would suggest a deck of cards and a first aid manual might be worthwhile additions.