gaskets on inside and outside
Just remember, you have gaskets that seal the slide when it is open and a set for when it is closed. Both of these need to seat properly, especially at the top.
And, of course, your RV must be level so that your slide roof is also level for proper drainage of rain water.
I had this problem on a motorhome I owned (2007 Monaco camelot) and it turned out that I had two problems; 1-my gaskets had hardened, and 2- the slide was not seated properly at the top-front of the slide.
2006 Fleetwood 35E Bounder
2015 Kia Soul - TOAD