Here's a really awesome DIY Slide On Four Wheel Camper Style Pop-Up Truck Camper RV by 'notesfromavagabond.' It's built on an aluminum frame he welded together himself and includes a few innovative features, such as a fold out room and a simple roof-lift mechanism.
(Thinking about building your own truck camper? (You can build them out of wood, too!) If you haven't yet, check out my article on How To Build Your Own DIY Truck Camper RV)