For charging without power, you can buy DC to DC chargers (I have some ISDT Q6 ones), which will take in a wide range of voltages on the input side (e.g. mine take DC 7-32V), and then balance charge up to 6s on the output side.
If you really have no option to charge your car from AC, then you can buy one large battery with a massive capacity (I'd say you probably want 3s as a 2s will drop below 7v before it hits the per cell voltage cut off) and charge from the big to the small one.
The C rating for discharge of the big battery isn't too important, as I doubt you will be charging at more than 2c on the output, so it's unlikely you will hit the limit of the battery before you hit the limit of the charger. but you do need to keep an eye on the input voltage to make sure it doesn't drop below the cut off voltage for the battery, in my Q6 you can set the minimum input voltage to make sure that doesn't happen.
I don't know for sure, but perhaps you can use one of these (edit: or something similar)as the "big" battery. Specs certainly line up, just have to have the correct cable.
https://hobbyking.com/en_us/turnigy-jump-starter-t13-mobile-power-station-20000mah-uk-plug.html