Lack of maintenance is the primary reason campers leak. Campers are basically houses that you're constantly trying to shake apart. If left outside they need to be recaulked every year or two, with proper RV-grade sealant like Dicor.
Usually the fascination with camping will wear off and the camper will end up parked in the back yard and neglected. After a couple years they start leaking, and it's all downhill from there.
Every repair project I've heard or read about has ended up being much larger in scope than what was seen on the surface. I personally would not buy a camper with known damage, EVER. You need to be persistent, diligent and patient to get the repair done. I don't enjoy that kind of thing, myself. There are other things that I'd much rather spend time repairing.