This problem can range from having a damp carpet near the end of the slide after a rain, to getting a major shower while driving your rig.
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Even though you have a fabric topper on your slide, water can still get under it, especially if there's a wind blowing. There's often a low spot in the center of the slide roof. Water can pool there (under the fabric topper). When you bring the slide in, the water can run forward, (especially if you're moving and you hit the brakes). The first time it happened to me, I looked like the winning coach after the Superbowl.
There are some things you can do to prevent it. I used to go up on a ladder with a long window squeegee and pull out almost all the water before bringing in the slide.
Later, I found that if I built a little diagonal dam with blue 3M painter's tape on the front inside corner of the slide, I could get most of the water to run off instead of pooling on the slide roof.
Take an umbrella out and get up on a ladder when it's raining. Watch how the water flows off the ends of the slide topper and see where it goes.
The tape is great for other things as well. It's completely waterproof, sticks well to most clean surfaces, and you can leave it on for weeks and still get it off without leaving anything behind — infinitely better than duct tape. It's good as a temporary patch for roof leaks and cracked light lenses, and for holding wires out of the way while you're working or hanging things while you're connecting wires. Important: If you have wood-grain wallpaper (not real wood) on the inside of your rig, don't put tape on it. It can lift off the pattern leaving some ugly white spots.
If you have a slide like mine where the roof slopes toward the rig when it's fully extended, the problem is worse. You can check this by leveling the rig, extending the slide, and putting a small level on the trim piece at the bottom of the slide. If yours is like mine, it can help to not put the slide quite all the way out. That can tip the roof the other way. If you do that, be sure to check the inside seal against the wall by the driver's seat. You want to make sure it's touching the wall at both the top and bottom. Another method is to mislevel the rig slightly to get the water to run away from the rig (if you don't mind pencils rolling off the dinette and water pooling on one side of the shower floor).
A tip from Ed Williams: "NEVER bring your slides in with the windows OPEN. Don't ask me how I learned this one."
When you're bringing in the slide after a rain, stopping it when it's at its maximum tilt away from the rig and waiting a few minutes for the rain to drain off can also help. You should also stop it often on the way in to give time for water to run of both the slide roof and the topper.
Another way slide leaks happens is when there is heavy rain and a strong wind from just the right direction. It can push some water past the seals. It helps to lubricate the slide seals periodically. The idea is to keep them soft and supple so they'll seal well. Thetford makes a spray foam for this, but some folks swear by silicon spray lubricant, which is significantly cheaper.
Something else to check is the rain gutter on the top of the rig on that side. There's a small piece of foil that covers a gap about half-way back. It can get damaged by low-hanging tree branches or overly aggressive cleaning of the gutter. On older rigs, the glue holding it in place can dry out and let go. If it's damaged, it can let water that's supposed to be going off the end drip onto the slide in the middle.
Of course, in any case, when you're expecting rain (especially with a stiff wind), bringing in the slide before it rains will prevent the whole problem.
If you do get a wet carpet, blot up as much of the water as you can with paper or cloth towels and aim a fan or hair dryer at the wet area until it dries. If you can get under the carpet, putting something under it to prop it up off the floor can make things go faster. Plastic water glasses work well for this. If there is a cabinet at the end of the slide, be sure to check the carpet inside it. Spray that prevents mold and mildew is a good idea for the carpet in there (even if you haven't had the problem yet). I like Moldex for this. It prevents future mold and mildew and it smells much better than the alternatives (in fact, it smells great).
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— Bob Ray