While 12v solar battery maintainers won't charge your battery quickly, they might be safe to leave unattended. If you have a 36v battery, I would string 3 regular solar chargers together to get the volts needed. Of course, the solar idea does you no good if you happen to get to your campground late in the day. An hour for lunch wouldn't give your battery much of a boost neither.
Short of a pocket nuclear reactor that folds into a utility tool, maybe a hydrogen fuel cell is what you're wanting?