You could find a 750-800W generator along with a charger that does not need more then 6.5A input. The Meanwell's are good quality, perhaps pick up a 48V unit version if thats what your battery is. You could place the MW's in series to obtain a higher voltage.
Otherwise you have to start hunting for power outlets and make note of them so you can charge up when you need to. Maybe make a Google Maps account and mark the locales. Gas Stations are good, Coffee shops in strip malls, parking lots with heater block outlets at each stall, behind many buildings are outlets, churches, schools, restaurants, country stores, country churches, mills along the train tracks, weigh stations, highway rest stops with washrooms. Anywhere and everywhere, just start to look for the outlets.