The sidewall outside skin cracking at the slide bottom has/was a issue with many 5th wheel trailers even back as far as my '97 trailer.
Mine started cracking at the front corner. From the corner it worked down at slight angle. Drill stop the crack didn't help. Finally dropped by my rv dealer for their input. Service mgr gave me a 18" foot long piece of flat plastic molding.
I cut the skin all the way down (13") and sealed the crack with a sealer...then installed the molding with a sealer with two SS button head scrws over the crack. That was in '03/'04 and its never leaked.
Everyone noticed many 5ers came from the factory with flat piece of molding over that same place. I assume the factory cut the skin so it would stay under the molding.
My trailer had no frame stress issues.