Check the seals for cracks and tears and replace as needed. Also, they might just need to be conditioned as they will become brittle over time. Its possible that they are not pliable enough and no longer provide a seal when the unit is in. I would sit in the closed trailer during a storm to see if you can trace the leaks.
I am still not positive which slideout you have but I'll assume its one of these. You need to double check, however. I recommend that you consider having your dealer do any adjustments for you unless you are very comfortable with the idea of doing it yourself.
If this slideout is a Lippert Above Floor Sofa Slide than the manual is here
. You can read about the adjustment for the lag and stop bolts, which are used to adjust.
If the slideout is a Lippert Electric Slideout System
, than the adjustments are different. See Adjusting room so it seals in the IN position on page 11.