I think I had the same problem as you - I could literally hear water sloshing when I start and stop after a heavy rain. The water was in the door sills (pull the plastic up and you can see the wire harnesses sitting in the channel full of water).
Mine is a 2018 too. The problem was the drain hoses were NOT connected in the rear pillars. I pulled the trim pieces both sides in the rear and reached down in there to reconnect the hoses to the connector that drains out below the truck. Fixed it.
I also used a syringe to pull water out of those channels in the door sills. Maybe a better method but it got the water out and I was able to get it all dried up.
Mine is a 2018 too. The problem was the drain hoses were NOT connected in the rear pillars. I pulled the trim pieces both sides in the rear and reached down in there to reconnect the hoses to the connector that drains out below the truck. Fixed it.
I also used a syringe to pull water out of those channels in the door sills. Maybe a better method but it got the water out and I was able to get it all dried up.