Purchase a good quality rubber gasket cleaner, lubricant or conditioner from your trailer parts store, and apply to all sides of each gasket. Rubber has a tendency to stiffen and crack with use, so if you do this every time you store the trailer you should be able to prevent leaks around the gasket before they happen. Also ensure that no debris gets between the gasket and the sides of the trailer and extension. This could allow water to get in around the sides.
Remember that the gasket is the only thing between you and the outside, and the best time to stop leaks is before they happen. As you clean, inspect all around the exterior of each slide-out, particularly the top.This is where most leaks start. Ensure that there is no debris on top of it. Carefully climb on top of the trailer to do this. Do not step on top of the slide itself.
Identify the source as soon as possible if your trailer develops a leak. Damp carpets can cause odors and mold to develop. Though most leaks occur because of weather, some can happen due to condensation from the roof air conditioner or from the water tank. It is possible that the leveling of the trailer can affect the seals also. Re-leveling should be done annually if you are in a park. Before you call in the professionals, make sure that you do not have a simple problem which you can fix yourself.
Apply butyl flashing tape to correct problems, or as part of your maintenance.There are several brands of this tape that adhere very strongly to the painted surface of the trailer and to the gasket, and all are guaranteed to prevent leaks. Some brands also come as a patch, which can be spot-applied if you have identified a single leak. The patch can be put on by any handy person but, because of the aggressive sticking properties of the product, you should have an all-around seal be done by a professional.
