How big of a family? How much driving would you really be doing? Do you plan on keeping it after the trip?
In my opinion, your design wouldn't be big enough for more than 1 or 2 people for such a long trip unless most of the family will be sleeping in tents. Fuel costs, while high, might not justify the cost of doing such a mod (that's up to you, though). Along with that, there's the resale value if you plan on selling after the trip. Your modified vehicle could end up being worth more to the right buyer after modding it, but there's no guarantee. You could buy a 10-15 year old Class C RV, drive it for a year, and likely be able to get out about what you put into it. You could even add some aeromods to improve the mileage (which isn't quite as low as you think).
http://ecomodder.com/forum/showthrea...d-e-18151.html