I had the same problem several years ago. We were down at the bottom of Grand Canyon for over two weeks.
I purchased three 30 watt Digital Camera Batteries from
www. digitalcamerabattery.com
They were great as I was able to power my D2x camera, storage vault hard drive and laptop. I just had a special cable for each. (Used the same battery)
Having three is better than one big one as you could always lose one in the drink or ----. I kept mine in three locations on seperate rafts. Also, you are only carrying around 1/3 the weight, etc. I also powered some LED's for night lights.
This is certainly one soluton.
I think they also have done something with solor cell charging, but iI am not sure. Give them a call. They were very helpful to me.
Hope this helps, Dave