I’ve never camped without power so that I don’t know unless you have a solar charger. I have a rad city step through and it has the bottle holder in the normal spot so in this group it’s helpful if you also let folks know what bike you have so it’s easier to get the right people answering.
I use a small, quiet Honda generator to charge 2 bike batteries on extended camping road trips. You won't get much charge out of most 12V deep cycle batter charging a 36V or 48V battery with an inverter. It would also take a pretty big solar panel and inverter to get a charge in a decent time.
Most bikes have cage bosses that bottle cages can bolt on to on the down tube and or seat tube, but a lot of ebikes, especially step through or folders don't. You can find strap adapters like the Topeak Versa Mount or SKS Anywhere Adapter if you don't have braze on cage bosses. Alternately, these work as great handle bar water bottle, snack, and bear spray holders:
Amazon.com : Bushwhacker Shasta Black - Insulated Bike Water Bottle Holder w/ 20 oz. Bottle - Two Point Bike Frame & Handlebar Attachment w/Belt Loop No Screws Hardware Required : Bicycle Water Bottle Holder With Bottle : Sports & Outdoors
SOLAR! I’ve been doing the solar e-bike thing since 2016 and it truly is the best way to charge, camping or not. I recently added a couple solar e-bike charging kits onto our website, and more are coming soon...
I have a renogy 400 watt solar generator and. 100 waat solar suitcase. I have a solar cooler, solar light and solar stove. The generators (I being 2) can be charged from the car if there is no sun so there is never a risk in rainy weather.