I really really believe that the more space you have the more stuff you take! Pack light, Pack smart and you will be able to fit it in!
This is part of camping! I remember being a child and having to sit in the car and my dad packing the car around me! I'd have to sit in the car until he'd unloaded it all at the other end too!
My tenpence worth is very much like other peoples: take ONLY what you need, buy food and bits there (if you can), and spend your time packing the car efficiently. My partner just leaves me a lone for an hour, whilst I talk to myself and squish everything in! We only have a ford ka, 2 adults and have been away for the longest trip of 12 days.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.