I sound funny to myself saying this: Space Bags.
I'm not even joking, I use them. You can get a nice assortment of them online, for about $30 shipped. They're actually a decent quality product, and easy to use even for just weekend trips, all they require is a vacuum cleaner with a hose, which just about everyone has, as far as I'm aware.
I routinely overstuff mine, and they still shrink the size of our loads unbelievably, when we take our weekend trips to fix my Van at my father in law's house.
- We typically pack enough clothes for 3-4 days,
- our double queen air matress,
- 6 large pillows,
- a full bedding set, including a comforter, blanket, sheet set, and a "topket" which is a stupid little blanket that gets folded up at the foot of the bed in case you get cold in the night, you can just reach down and grab the top of it, and pull it up over you,
- all her school stuff (about 2-3 book bags full)
- several snacks and other things,
- some tools,
- her laptop and bag,
- a full size spare tire,
- a complete car-care basket,
- a hydraulic jack,
- and a 4-way lug iron,
all in a '94 4-door grand prix.
It all fits, and I can still see out the back window, and we still sit in the front seats more than comfortably, for our 3 hour drive.
Given all that crap, fitting in my wife's car, I'm sure your wife could find a way to pack up all her stuff into an xB with 3x the space of our Prix!