I know exactly what you mean. We also have a mini van that barely holds our family of 6. This Christmas we went to my family, it was a 15 hour drive each way. My boys were helping to pack the van, and my husband helped me get settled in. We were an hour away from home (the longest I can go without some kind of support) when I asked my kids to pass me my pillows.
Well, long story short, I doubled up on pain pills, my hubby did all the driving, and we stopped at every store that had any chance of having a pillow that would help me.
Drive safe. When Hubby is driving lots of deep breaths. (I don't know if it will help or not, but it can't hurt) LOL.
Have fun.