Currently someone could buy a mini or medium generator from Generac or Honda. Both companies make mini units that daisy chain together to supply more power output.
Mount/Attach them on a trailer hitch cargo carrier and you're good to go. Teslas won't turn on and drive while they're plugged in and charging but with this set-up one could charge up during any stop.
Edit, Personally I'd prefer a propane generator. Because I could use the same tank to power a camp stove or lantern. And at least in the US any gas station, hardware store, or "super store" has the cylinder exchange program.