If you have made the decision to buy a caravan, even a second hand one, it is important to understand that this is a very significant investment, and one that doesn't come cheap.
This is why, when viewing the used caravan you are interested in buying, you should take a few extra minutes to thoroughly inspect it from top to bottom to make sure that it is in the sort of condition that you would expect.
Use this checklist to make sure you're looking out for the right things:

• Damp: This is the most important thing to check in a used caravan, as it can cause hundreds or even thousands of dollars in damage if it goes unchecked.
  Inspect every inch of the van's interior, from the corners to the backs of the cupboards, to ensure that everything is dry.
  
• Doors: Make sure that all of the doors in the caravan open and close properly (this includes cupboards).
  Can you see any daylight around the edges of the main door? Do all of the locks drive home properly and hold the door in place?
• Floors: As you walk around the used caravan, have a feel of the floor and keep an eye out for weaknesses and sagging.
  This will be especially prevalent around the doors and main living areas of the van, and could be a sign of wear or (even worse) damp.
  
• Seals: There is a range of seals available for caravans, the cheaper ones lasting for around five years and the more expensive ones lasting up to twenty.
  Make sure that you check all of the windows, doors and panels of the van to see whether any seals are cracked or even missing.
  
• Windows: Make sure that all of the windows in the used caravan open and close properly, and that they have secure locks.
  You should also check the glass for condensation, as this could suggest a damp problem (unless the glass is double glazed).
  

By checking the above areas of your next used caravan purchase, you can rest assured that you are getting the sort of quality that you are paying for.
There are, of course, other areas of your caravan that you should be giving a thorough look over (such as the electrics, gas, stabilizer, brakes, breakaway cables, hitch and chassis), but the ones outlined above tend to cause the most trouble if issues go unfixed.
Good luck with your search!