When you "level" the Sprinter, the roof of the slide out should have a slight tilt or slant away from the body of the RV. This allows any water to run away from the slide and unit.
Perhaps your unit is sitting so that the slide out is slanted the wrong way. Try placing some boards under the wheels of the opposite side.
Adjustments can be made to the slide itself and directions for doing this can be found in the Owner's Manual from Lippert. Go to their website (
www.lci1.com) to download it but you also need to know if your slide out is hydraulic or electric motor operated.
Check the rubber seals all round the slide to make sure that they are in good order and "folding in" and out properly when the slide out is retracting or being extended. Also have a look at the bottom edge of the slide and check the caulking around the metal trim piece.