Here's a great review for the Cascadia Vehicle Tents. It demonstrates how the roof top tent is setup. This family was traveling from Alaska to Argentina with their 5 kids and using the roof tent for camping along the way. One of the biggest advantages of the roof tent is the quick setup and tear down. It also allows for comfortable camping on any terrain assuming you level your vehicle first.
This is an affordable alternative to the ARB Roof Tents. As a bonus they are local NW company based out of Bend, Oregon. These unique style tents aren't for everyone but sure are unique and the perfect addition to any expedition vehicle. Check them out: http://www.cascadiatents.com/