You are not going to get enough power from a small solar panel to run an inverter so some form of USB or maybe 12v charger is your only option. Can you get a car charger for the Nikon battery?
New camera batteries are all li-ion batteries and cannot be trickle charged in the same way as a car battery can. You either have enough power to charge them or you don't.
Can you get a AA adapter for a Nikon? (Canon allows you to use AA's in the hand grip but not in the main camera). The AA metal hydride batteries can be charged from a solar panel without too much problem. I'm not sure how many shots you get from a pack of AA's, for Canon you need 6 AA's and you don't get many shots.
You don't inherit the world from your parents, you borrow it from your children.
1979 Unimog 416 Expedition Camper
1974 Unimog 421
2004 Dodge Ram 2500, 4x4, Double Cab, Cummins Turbo Diesel
2006 25' Airstream International CCD
2009 Harley Davidson
Sugarloaf, Boulder, CO