A few years ago I went shopping for that very thing. I found the best set-up to be 2 separate items...a charger that would work on 12VDC, and one of those roll-up (fabric) style solar panels. The hardest part was to find the right charger. Sorry, but I don't remember what products I settled for, but I do remember that I found both items on amazon.
They may have come out with better technology/packages by now.