If your prospective RV seems to have a few screws too much, ones that don’t even follow the pattern of the rest, it’s a good sign that the roof has been flooded with enough stagnant water to sag. Moreover, look at the underside of your RV for any signs that the floor might have been patched up or welded.

A good way to ensure that you don’t end up with a lemon of an RV is to rely only on reputable sellers like Johnson RV. These companies ensure that every vehicle that passes through their sell my rv service is of good enough condition for you to buy. 

http://www.johnsonrv.com/blog/post/2014/08/06/Inspecting-Used-RVs-for-Sale-Checking-the-Vehicle-for-Wear-and-Beyond.aspx