Dealer says water damage, huh? Water damage on a piece of "wood" that's a good 6" in from the side of the trailer, AND at the floor... I'm going with Radar57 on this, impact damage. The slide out is a Schwintec and I'd bet the slide got out of timing and went out crooked and the something on the slide caught it. When they are in time there's barely enough clearance, Clarence.
OR, the other possibility is whatever is riding on the black track that's screwed to the floor next to it got a little crazy and slapped it.
I'd be looking that entire unit over with a fine tooth comb, and have another set of eyes with ya, preferably one that's been thru getting a new unit before, and if that's the only thing wrong and they are offering at no charge an extended warranty, bumper to bumper, I'd prolly pull the trigger.
Hopefully that's all that goes wrong and you got it out of its system early in the game.
2017 North Point 375BHFS
2015 Silverado 3500HD Crew Dually Dmax 4x4