Camping? By bike, car, camper, other? Are you sure you'll have no access to mains? Touring by E-bike is becoming popular, so people must be finding charging opportunities. If you do need extra capacity and it needs to be portable, an additional battery/s is probably the simplest (EDIT - yoav beat me to it). Extra batteries weigh about 3kg each. Some tradesmen use little petrol generators to charge power tools, they're bout £400, though there feels something wrong in charging an E-bike by burning petrol...
If you're in a motor vehicle, than a leisure battery and inverter might be worth investigation, though apparently you have to be careful to get the correct type of inverter for charging. I've only a vague idea how that works, I had such a system in a van, but it was fitted for me (Heavy duty alternator, the leisure battery only charging when the van battery was full, the inverter only drawing from the leisure battery)