? Charging Electric Bike Battery

Without getting too complex here, if you're going to charge it from the truck, I'd get a pure sine wave inverter that clamps on the battery, not uses lighter socket. Because the battery is 36V, and presumably the amp hour rating is at 36 volt, my thought is the energy equivalent is about the same as a 45 amp hour 12 volt battery. So it will draw down the truck battery quite a bit to do a full charge. Your inverter rating would depend on the charging rate of the charger. Plus a decent safety margin.

I agree with others, if you can scrounge up other reasons for a small generator, why not? That little bike battery could have at least half the capacity of the truck battery (depending on what truck).