As noted above, a lot of things could be done to improve the MPG
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Rolling resistance is going to be high due to the weight of the RV, so, see if you can get lower rolling resistance tyres, and pump them up a bit more (but, not crazy-high as some of the cars on here, as you don't want to blow one of them on a vehicle that size).
Front (if they wheels on full lock don't protrude from the wheel arch) and rear wheel skirts will also help, as will a full belly pan. These would have to be removable, at least in the areas where you need to change wheels and tyres.
Finally, the cone mentioned above is a good idea - if you had enough time you could design a cone shaped attachment at the back, which if made well enough, could even be used to store (lightweight) items while you are travelling around e.g. spare bedding and the like.
With the above mods, which won't really affect the appearance (apart from the cone, but, if done well, this would not look odd at all - you could have a 'square cone', to more closely match the style of the vehicle), you should get a big improvement in MPG!.
You could also try the trick of disconnecting the fuel lines (after depressurising them), and having a small container of diesel. Idle the RV on that, and see how long it takes to drain it, then work out the fuel consumption at idle. If it is very low, then you can engine-on coast whenever possible, to save fuel that way, and reach the top of hills at 20mph (if not holding up traffic), so you can then roll back down the hill and be at 60 by the bottom of the hill - that RV will have a lot of momentum for this kind of driving!
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