I would think a roof spoiler like what is used on semi trucks would help your mpg. Based on the camping rig in the first post. Something like this:
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Then something like this on the rear:
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Or you could fabricate a full aero package like aerohead's image:
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Lastly, I have a standard sized Chevy Astro camper van as a daily driver. I'm getting around 28mpg right now (when not hauling junk) and its still got all the factory RV accessories like AC power, bar lighting, fold down bed, toilet, etc still in it. I'll be over 30mpg in the near future thanks to the advise of this forum! I don't know your situation. But picking up a smaller camper van like mine could save some cash, if the smaller size works for your needs.