If you search the forums for a thread titled "Nebraska ate my Prius," I recount my tale of a cross-country trip.
In short: We had four adults, one infant, and a full trunk of gear. While I never did weigh the gear, the passengers were a total of about 800 pounds, and the trunk was quite literally completely full, with the 'hideaway compartment' lid removed, and sliding shade removed, and packed by a Tetris expert completely to the glass. I'd estimate a conservative minimum of 100 pounds of stuff. (Plus the gear packed in the back seat's footspace in front of the baby, and a few assorted personal bags in the footspace of the various passengers. Purses, my laptop and camera case, etc: i.e. things that were needed on the drive itself.)
This was for a 3000 mile each way trip. Each half driven nonstop, with a week in between coming and going. The only problem was when a tire disintegrated on a really nasty gravel road. (I suppose overloading could have contributed, but I did have the tire pressure at Toyota spec, instead of my normal +4 PSI.)