The slide seals are supposed to prevent water from leaking into the trailer. You may have an issue that the dealer should address.
Some setup their TT with a slight tilt downward on the slide side to make sure rain runs off away from the TT. A slight tilt front to back using the tongue jack can also be helpful in encouraging rain to run off rather than into the trailer.
There are many threads on this forum about slide awnings. Supporters note that the slide roof stays cleaner and dryer, and they do not hear noisy raindrops hitting the slide roof. Others say that debris can blow under and be trapped beneath the awning and that windy conditions can flap the awning creating a lot of noise.
I try to keep things simple and either sweep or hose off the slide roof prior to departure.