Message boards are full of people miscalculating MPGs, lying about MPGs, having a strong confirmation bias about improvement of MPGs and not taking into account varying conditions while calculating MPGs.
You're extremely unlikely to change MPGs any significant amount (+/- 1MPG) without making major concessions with your design. For example, a roof so low in the 5th wheel than you can't stand up. As people pointed out earlier, slowing down makes the biggest difference by a long ways. I also agree with some others that RVing isn't cheap. For all the things we pay for without giving a second thought, RVing should be one of them. Just fill it up, take the receipt and have fun. Skip Starbucks a couple times a week if you really want to save money :)