We just got back from a camping trip and had same concerns. I have a solio solar charger, but it doesn't work with iphone, even though it should.
I took three spare battery packs for my camera, that was more than enough for a week long trip, I only used one pack.
Fully charged iphone lasted 4 days with minimal use. We just used GPS and looked up a few websites, no games, no music, no phone calls, no emails. Then we stayed at an inn and charged it. It was actually nice to unplug for a few days.
Depending where you camp, there's usually an outlet in the bathroom, but it sucks to sit around waiting for the phone to charge.