Michael,
Your idea of tilting the slide down on the outer end is possible to a slight extent, however you probably will not be able to tilt it enough to solve the problem that you experienced, especially with the coach not being totally level. Water leaks are very difficult to locate, and sometimes they are difficult to repair, but finding the exact source of the leak is usually the hardest part of the repair.
With seven year old seals on the slide, they will probably have to be replaced if the slide is adjusted to change the way that it fits in the opening. My bet would be that the outer slide seals need to be replaced now, however that is a call that needs to be made by someone that is experienced in repairing water leaks on this design slide, if the slide is really the source of the leak.
Gerald