When looking at a rig, be sure to check out the slides. You want to operate them and watch for how smoothly they come in and out. Once they are out, get outside and check underneath. Press against the bottom looking for soft spots. With the slides in, examine the framing around the slides. Water leaks will often leave the framing soft and spongy. Although not structural, it could lead to mold. Check the deals around the slides both with it in and out. The slide should seal completely both ways.