Here is a quick list of basic this I would recommend looking at.
I will try and add more items & more details to the list as I have time.
USED CAMPER CHECK LIST -- BASIC THINGS TO LOOK FOR:
Check to make sure all of the roof screws are tight, if not hand tighten them and re-seal them with a dab of marine grade silicone once you get the camper home.
Check to make sure the roof does not have any holes or tears in the exterior aluminum roof skin (from trees or misc. items poking it over the years).
Check the soft sides (pop-up up portion) of the camper for unusual wear, tears, or large holes. If there are just small pin holes, those are pretty easy to seal up or patch if needed.
Check the inside of the camper roof (head liner) for rips or tears, stains from possible leaks, or for mold.
Check the front and back wooden folding panels (hinged panels that allow the roof to go up & down). Check to see that the rivets are intact and that the wood is not rotten or wet.
Take a look at the aluminum skin all the way around the outside of the camper. Check for scrapes or punctures.
Check the exterior wooded camper base (usually it is painted grey) for any abnormalities.
Check the eye nuts in the camper base (outside of camper) to see if they have been over-tightened or might be pulling away / pulling out / crooked or bent.
Check the propane box and see if everything looks as it should. Not many things to go wrong with this.
If possible, have the selling turn on items for you.
Example: see if the lights work, see if the furnace lights, see if the refrigerator lights, see if the water pump turns on, etc.
This might not always be possible, but it is always good to poke around and play with things in the camper for a few minutes if you have the opportunity.
Check to see if they have the keys for the rear camper door.
Check to see if the camper jacks look solid and not too bent.
Check to see if the awning cranks or pulls in & out properly.
Check to see if the roof vent opens and closes and does not have any cracks in the lid.
But the best thing to do is just spend a few minutes looking over the camper, sitting inside, smell check inside to make sure it does not have a "funk" to it.
Does it look like the owner of the camper has taken care or the camper ?
Or is it hammered and beaten on ?
Has the seller made any attempt to clean the camper out and make it look presentable for the new buyer ?
Does the camper fit my truck ?
Check the camper fit guide here, or call four wheel campers ...
http://www.fourwheelcampers.com/camper_fit.htm
If I were looking to buy a used camper for myself, I would look at the following items above, and then I would probably just sit inside the camper for a few minutes and ask myself ... "will I be comfortable camping in this camper? Does it look nice inside ? Does it smell funky? It is just dirty & gross?"
Or are you telling yourself ... "Hey, this camper is pretty nice. It is clean. It looks to be well taken care of. Doesn't smell. Yes, I would probably be comfortable camping in this camper".
Give it the "Feel Good Test", lol.
Hope this helps.
Happy Camping & used camper shopping !