We had the same problem and went back to our RV shop (still under warranty at the time) and they found the slide was missing "drip cups" on either side of the slide. Once they put them in place it sealed the leak.
Jeff & Naomi & Ryu
2019 Thor Chateau V25 Class C SKP: 154598, Good Sam: 813449980
Upgrades: 190W Go Power Overlander Solar Panel, EPever 40A Tracer MPPT Controller, Xantrex Freedom XC 2000W 80A Inverter/Charger, 4x6v VMAX AGM 240Ah Battery Bank, SeeLevel III Tank Monitor, Big Foot Leveling Jacks, Jensen CAR1000