I want to tour the US and tent camp my way through, so I won't have a terribly long time each day to keep my GPS, phone, camera, etc. charged... so are there any bike mounted generators that can tap into the drivetrain or something to generate a small amount of power consistently to keep handheld electronics charged?

I don't need to charge a laptop or anything, just small lithium ion batteries. The ability to plug it in directly would be best, I think. Instead of storing power and charging at night, that way I could charge as I ride and swap each out as needed.