iPhone battery life is not great. Partly because people use the iPhone so heavily, much more than Blackberries etc (the carriers report than bandwidth usage is 3-4X higher for iPhone users than for other smartphone users, and bandwidth i.e. transmit/receive is what burns up the battery). And partly because its not a big battery.
To max battery life, turn off WiFi, set to Fetch not Push. Easy enough to turn WiFi back on when you want it.
If you're going to be in a van for hours, carry a $20 car charger and if needed a USB extension cable (if you're in the back seat).
An extra battery is not a bad idea. Check iLounge
All things iPod, iPhone, iTunes and beyond | iLounge reviews for the best one. There is also an iPhone case that has an external battery built in.