Make sure you run the slides in and out before buying. Two reasons come to mind:
1. You want to see what it looks like with the slides in. When traveling, you may want to stop and have lunch or just use the bathroom. Can you get to whatever you want to use with the slides in?
2. On a used rig, there could be a problem with a slide that caused the former owners to sell. Listen as the slides go in and out. Any "weird" noises? Especially watch for "jerking" or irregular movement (anything other than smooth and steady). This could be a sign of a problem that will be expensive to fix (ask me how I know).