Keith,
I agree with you that if there if there is a gap the water will come in, but only on the side seals. The top of the slide is a big trough with about a 4 inch wall on that stays on the inside of the coach and another wall on the outside.
From the front or back, it looks sort of like this: [___________________] It is a big wide gutter, but it has several seams and rivets that leak if not well sealed. Any water on the slide tops should flow off the ends. But, depending on the wind, rain intensity and coach levelness, some water will get under the toppers pool on the flat metal slide roof. If there is a seam or rivet in that pool, you have a potential leak.
To stop the slide roofs from leaking, I had to get all three sealed where the factory failed to get it right. The inner corner is very difficult to reach. The rest is not so bad if you can figure out how to get the topper out of the way.
The vertical side seals are another story. The slide is supposed to smash a rubber strip enough to make it water tight. My experience was that depending on rain intensity, wind and levelness the large flat motorhome roof would collect water and it would have to flow somewhere. Sometimes it was toward a slide where the seal might have to deflect the equivalent of the flow coming out of a garden hose. One day at Ft. Flagler I stood outside for about an hour in heavy rain watching a significant stream of rain water flow off the roof and attack a seal. Some of it was getting inside and making the bedroom rug wet.
The slide had been "professionally" adjusted under warranty to stop the leaks. My solution was to install a gutter and some "deflectors" so that, no matter how hard it rains, the roof water is forced to flow somewhere other than a slide. If may still be less than a perfect seal, but now the only water that falls on the seal itself has to be just a raindrop itself! And, that is not enough to make for a noticeable leak. I no longer allow any streams coming from the roof to flow directly on to a slide seal.
