Most silde-outs have a strip of rubber seal along the bottom of the trailer at the floor/ slide junction. If this seal is missing, folded under, or buckled it will allow water spray from the tires to blow under the slide. If your trailer does not have such a seal, there must be another type of water/weather deflection.
As already said, the water may be entering anywhere from the top of the slide down, to and including the bottom.
If all else fails, you may consider buying some seal(style to your liking) the same length as the slide-out floor, and installing it on the bottom of the slide, placed so when slide is closed, the seal overlaps the trailer bottom.
2000 Winnebago Ultimate Freedom USQ40JD,
ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG 11B5MX,Infantry retired;Good Sam Life member,FMCA." My fellow Americans, ask not what your country can do for you, ask what you can do for your country. John F. Kennedy