I think Frank Lee has listed most of the low hanging fruit often found in other threads, this would be the least invasive route and the most affordable.
Remember, the larger and more ambitious the project, the more risk involved, mostly the risk of not finishing and ruining a completely good camper.
One suggestion even more obvious is to drive to shorter destinations, and more slowly. However, that's going to take most of the adventure out of the equation.
These are nice, and appear to have some aero-consideration.
http://www.yotatech.com/f123/holy-co...yota-rv-58777/
In 1982 I test drove a home made camper based on a early Japanese import pick up truck.
It was very heavy, the guy had used plywood, coated with fiberglass and painted it.
I could not keep the truck going in a straight line on the highway, it was terrifying and exhausting.
I ended up with a full sized short box Ford van (E-100 in orange - I later painted tiger stripes on it).