People will often advise that you have the vehicle professionally inspected. While this is a good idea, I have often found it impractical because it is difficult to find a mechanic who is immediately available to do an inspection and knows what a pre-sale inspection consists of and has a set price for it. Since I couldn't find a suitable inspection mechanic, I made my own inspection. Start your inspection by looking at all of the RV's surfaces and make sure you can live with any dings, dents, scratches or stains that might be there. Next check for leaks.
Leaks can be very difficult and costly to fix.
Look for any stained or warped walls, especially in the corners. If your RV comes with a generator, make sure it starts smoothly and measure the voltage it produces. It should produce around 110-125 volts. Check if all lighting fixtures, especially florescent ones, come on at full brightness. Turn on any fans to see if they need lubrication.