On a family camping trip (2 adults and 2 small children), we fit all of our gear into the trunk and back seat. It took some creative packing, because the Prius' trunk is taller than my previous cars' trunks but not as long, so I had to learn to stack and pack in layers. The following pictures show everything in each layer.

Top layer: a suitcase, cooler,
collapsible water jug, blanket

Second layer (on the ground with the top layer): Coleman lantern,
bucket of miscellaneous gear, and another suitcase

Third layer (in the trunk): tarp, portable crib, sleeping bag, huge canvas tent

Fourth layer (on the ground with the third layer): tent poles,
two inflatable air mattresses, another sleeping bag, air pump

Inside the car (under the boys' feet and alongside their car seats)
we packed a bag of groceries, two diaper bags, some towels and sheets,
a lot of toys for the boys, a small lawn chair, and a folding high chair with tray.

The complete campsite!