Welcome to the forum and sorry about your first trip being a "duck soup" kind of morning
There are a lot of reasons why a slide might leak and not all of them are related to bad construction or improper assembly.
You have a wide rubber "flap" on both sides and the top of the slide, that "flap" folds in when retracting the slide and is SUPPOSED to fold out when the slide is extended. This is not the "slide seal" but is a weather diverter. Its function is to lay against the slide and as water runs down the side of the RV, it hits the "flap" and is diverted away from the seals.
If one of the corners didn't fold out, water could run into the RV from that spot. If the "flap" didn't fold out when the slide was extended, it would "invite" water into the RV.
I'd suggest you extend your slide, then go outside and look at the wide black rubber "flap" to make sure it's folded out and is in the correct position. If that's not the case, then maybe a call to the dealer to have him check it out !!!