If you look at the Mazda5 then also look at the Kia Rondo. If I remember correctly, the Mz5 doesn't have a perfect floor when you fold down the seats which makes is harder to sleep in the back. My buddy has the Rondo and his floor is perfect plus he took it one step further. He built up a bed for the back which you can still store boxes under, its a pretty sweet setup and if you do a lot of trips where you end up sleeping at a trail head, its a great setup. I"m sure the same thing could be done with the Mz5 though.
Also check out the Hyundai Elantra Touring, there is a lot of space in that wagon.