I have often camped in a 139 when the temperature is in the low 20s (fahrenheit), but that's as cold as we can go. Otherwise, we risk freezing the lines. And even the low 20s is a little risky.
We go off--pavement all the time, but very slowly. We have added 14 inch tires, load range D, and are very careful going over potholes and rocks. We also have an "axle flip," which gives us extra ground clearance.
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)