If it has been going on for some time, there is a good possibility that some of the wood on the slide has deteriorated. It might be best to pull the slide and fix it right, including all that has rotted. You can usually look under the bottom of the slide after it is extended and see if any of the wood has gone bad.
On my Monaco the canopy over the slide forms sort of a valley where the water coming off of the roof is supposed to channel to the right or left to get off of the roof. I would guess that is where the leak is. The problem is that you may also have damage in the wall above the slide as well as in the bedroom floor and the only way to find out is to remove the slide.