Awesome! Would love to see a few pics (once you're allowed to post them)! Do you attend the Bay Area Maker Faire? I know my high school robotics team would be really interested in seeing what you've got going!
In any case, I'm biased toward Diesels so if you're willing to do an engine swap (and can find a good used diesel engine that will work with your drivetrain), then I'd go with that. As far as body modifications, there are a wide range that could help you streamline your camper brick enough to improve your FE significantly... from mild to wild... as most on this board will tell you, start with the rear (boat tail or something similar) and underside (belly pan) first and then improve airflow on the rest.
If you want to go all out, you could do something similar to the Texaco Doodlebug:
Obviously with smaller wheels/tires and maybe with rear skirts...
