A few questions to try to isolate the problem:
-Was the slide in or out when it leaked?
-I assume you do not have slide toppers. Correct?
-Was it FULLY out (or FULLY in)?
-Has it been in rainstorms before, without leaking?
-Have you brushed that side up against a tree or other obstacle recently? (No matter how gently)
-Are you SURE the windows were closed tightly?
Note that getting pissed may make you feel better, but it will not provide a solution for the leak.
As suggested, inspect the roof of the slide for rips or scuffs. Also carefully inspect the seam where the outside wall meets the roof. The slightest crack in the calking will allow water to get inside the wall and then run down. Remember that you are dragging a house down the road. Things flex, vibrate, and twist. Cracks happen.
If all else fails, take everything out of the cabinets and get in there with a strong light while somebody flows water from the hose on the outside and top of the slide. When and if you see water, trace it around to the outside. Have some towels handy. Stopping the water outside will not immediately stop the water inside.
"Popout" must be a Cali thing. I have never heard that before.