Looks to me your floor is getting wet and the simulated molding is wicking the moisture, destroying it from bottom on up. You may have water running down the exterior wall of the slide and coming in wetting the carpet. Then the molding soaking it up.
*Current: 2005 Ford F350 Crew Cab Dually 6.0 diesel 4x4*
*Retired: 1987 F350 Crew Cab Dually 6.9 turbo diesel
2007 Forest River WildCat 32QBBS