Start with this
ScanGauge MPG gauge: improve your fuel economy - EcoModder.com
You probably won't need a 4x4, you would be supprised where you can get 2 wheel drive. You just have to learn what your limits are and how to weasle it into whereever you want to go. I have gotten my 2 wheel drive 4cyl Frontier into places that made people with fancy v8 powered SUV's jealous. That and most campgrounds are pretty well developed.
Most trucks and cars have an upper and lower grill. You should be able to block off at least part of the top one, and with the scanguage you will be able to monitor the temperatures.
Switch to an electric fan if it has a mechanical.
Use high load rated tires with a high sidewall rated pressure. If your going to be actually going off road see if you might looking into BFG XO's. Depending on size they go up to 60psi. I dont think they make anything below 30" tall though.