You would be much better served buying a ~7-10KW inverter type genset, running it on "home brewed" CNG, Compressed Natural Gas. Mount it on a small single wheel, swivel wheel, "tagalong" trailer.

Continuous (re)charging of the hybrid battery without the wastefullness of the HUGE ICE just to "trickle" (in comparison) charge the hybrid battery.

Improve the hwy MPG by 100% even counting, including, the cost of the CNG.