Doug,
I am a little confused. Please tell me how rain on the topper can get in to the coach unless the topper fabric has deteriorated to the point where rain runs through it. On my rig the topper is mounted on the exterior above the slide and as I retract the slide, any water on it will spill off the front or the ends of the fabric and NOT get on top of the slide. In other words my topper is large enough that it extends over each end of the slide so water draining off of the ends of the topper will just drip on to the ground and not on top of the slide. I also have a rubber seal/wiper type gasket on the top and sides of my slide that will sweep off any water that does get on top of my slide although I have never accumulated water on top of the slide itself. I recently changed the fabric on one slide topper and it was the same fabric as I have on my main awning and water does not run through it when it rains.