When slide extended, make sure seal/ wiper is all facing out? Slide AWNING HELP REDUCE leak risk on top of slide, but always safer to retract during storms/ bad weather. Also while extended, see if slide centered front to rear? Mine was not, and I had to adjust when I replaced a torn vertical wiper seal. Several problems I found in my then 17yo rig were mostly caused by techs/ repair (maybe DIY owners?), and I DIY repaired all.
Sometimes, when slide extends, most but not all of seal extends, and water will flow to inside where seal did not flop out. If you extend and then inspect, you can pull out any in wrong position. But if slide not centered, it can also rip seal loose, or tear it.
2000 Conquest LE 6266, Class-C on 99-E450SD, 6.8, 2v, V-10