I was working through a concept of a Honda 1000W gas generator, with a transformer and rectifier that held the voltage just above the trickle charge voltage for the battery pack. This would all be put on a trailer for longer tours.
I did not have the cash to experiment but did some calculations and prelim parts sourcing and "speccing". Based on my figures, it would work if the Honda voltage regulator could hold the ac voltage in a narrow range under load. One down side is that the transformer coil is relatively heavy, although it could be engineered down to "as light as possible without burning up".