Your problem can be related to floor rot, but there is metal rail that slides on a roller, if the roller is damaged or missing is will cause the staining on the rug. Some that I've seen can dirty the carpet that everything is just fine it's just caused by dirt coming off of metal rail. Back to the slide problem you need to look at it from under the trailer. If like most it has a plastic looking cardboard sheet screwed across from frame to frame underneath that you need to remove so you can look at the slide mechanism to see if the wood floor has rotted where the metal support attached and what's left of the floor hasn't fallen down. May not have explained it right. But you need to look underneath first to see what's wrong. It kinda hard to diagnose without seeing it.
Retired Professional Firefighter
2007 Nuwa Hitchhiker DA329RSB