Water damage is by far the #1 thing to inspect for.
The newer the camper the less of an issue this will be. Caulking on the exterior needs to be replaced as often as every year, maybe it can go 4-6 years, but it does start to crack that first year. This is where water gets in 99% of the time. Check the seam where roof meets wall outside, and all the penetrations like roof vents, sewer vents etc. Same goes for the exterior walls. If the caulking looks original then inspect a lot closer. If it looks like it was just done then be a little more weary that they did a hasty repair at the time of selling.
The "caulk" should be Lap Sealant, not caulk, not bathroom caulk and NEVER silicone in any form. Silicone will be a nightmare for you when it is time to do work. Silicone works quite well but will be hard and impossible to remove.
Bring a multi-meter and check the battery charge with the camper not plugged in. If it was plugged in, unplug, turn on lights or something to put some drain on the batteries and test at the end. 12.6 is a healthy battery.
Sniff around cabinets for mold. Look for black mold and run if you see any. Campers might smell musty if closed up a lot, but should not be moldy.
Check that the holding tanks are not cracked. Fill them up if needed. Sometimes people leave the poo in over winter and that cracks the tanks (the water does). Cracks are not a big deal to fix depending where the crack is though.
Some things will be wrong with the camper a lot of times, so be prepared to do some fixing up and know your limits both on ability and available time.
**edit. Ask about the refrigerator and what precautions need to be taken. This will give you an idea of what they know about use, so you know if they even knew to take the precautions. If they do not know, then they have likely done damage to the absorbtion unit..
I dont want to give out the "correct" answers for "sellers" so just say, go research this yourself.
