Before you buy a R.V. here are a few things to look out for.
First you need to have a good close look at the roof and ceiling, especially If the coach is close to ten years old or more . If the roof is rubber you need to look real close and see if it is cracking or it is starting to turn black or the wood under the rubber is starting to raise or get bumpy, if this is the case, get out of there or negotiate a lower price after getting an estimate for repairs, also look at the ceiling and look for any water stains or water rot along all corners and along every edge and seem. make sure it is not freshly painted as to hide stains.
You also need to take your thumb and push on the wall paneling below the corners of every window so as to check and see if it has any water rot there. Also look for water stains on the back of all the curtains or shades. recent replacement of curtains are also a tail tail sign.
If the coach your looking at is a class C, don't get lazy here crawl up over the cab and check under all the windows and under the mattress for water damage
Make sure you have the seller get the refrigerator running at least a day before you go to see it, as it take 24 hrs for an RV refrigerator to get down to temperature. If you smell ammonia around the refrigerator or it doesn't cool you could be looking at a $1500 replacement.
You also need to inspect the floor around the toilet and under all the cabinets (especially were the water lines are run), if they have it loaded with pots and pans ask them if you can move them to have a better look many times people have a slow leak that goes unnoticed and rots out the floor and it is very costly to repair.
It is always good to have all the holding tanks filled past full to check for cracks and leaks but if they don't have a dump station have a good look around there tanks at all connections and look for cracking and evidence of leakage.
You also want to have them unplug the coach and see if the lights stay on inside to see if the batteries are any-good, see how the lights look after 10 min or more without being plugged in. Then again its always good to throw some new batteries in when you buy a new unit and plan on dry camping.
These ate just a few things that you can do to save yourself some big headaches down the road but your still best off having some one inspect the coach before you buy. If it is a motor-home have the drive train inspected by a good mechanic.
If the seller is watching you look things over take your hand across all the things you see ass a problem or imperfections so they know you see them and it should help in the negotiation of the unit.
Hope this helps you, and happy RVing!