A prevous thread a year or more ago asked for where to find a kit to convert a van. The "conversion" wanted was the pop-up or elevated roof used in some of them. After some research by quite a few people, the conclusion was that no reputable manufacturer would sell one of the availablle "kits" to anyone other than a professional conversion factory or "custom shop."
The "shell" of most modern vehicles, including vans, is at least a "semi-monocoque" construction that uses the skin as an essential part of "what holds it all together." Once you cut a hole in the skin, it is extremely difficult to add on enough "other parts" to recover anything close to the original strength.
Putting "gear" inside a van is safe enough for a DIY project, provided that you don't grossly exceed load capacity of the vehicle; but that's a "van equipped for camping" rather than what's commonly called a "camper conversion."