Cost effective is already present. A TD tow vehicle and an aero trailer. One can be in the 20's without the years of building and testing.
The "cost" is nights aboard over years of use. The truly economical is the one that lasts decades. Miles is mainly a function of affordability. Whether one one moves once monthly and then maybe 200 miles or wants MORE has to be weighed. Most retirees RVng cite $2500-3000 as a minimum income. Less than this is not the point it is that the upfront costs should be done while one is in a career. I'd say it's hard to reinvent the wheel and not particularly effective to do otherwise. Modify existing maybe but keep in mind RV capacity (water and propane and food storage) mean more than RV mpg. It is he solo miles of a combined vehicle that the real savings occur. No moho will ever meet the same test. It is a mobile house and not an oversized car.
Here's a post (along with the link above about the boattail Class C) that may whet your appetite for building:
http://ecomodder.com/forum/showthrea...tml#post456738