Well here is the thing, the rubber is usually alright but the steel strip that holds it to the flange part of the cab/sleeper rust and because of that, it pulls the rubber away from the flange to let water in.
If the rubber isn't ripped anywhere (which is usually at the bottom because of foot traffic) you can seal it up with good silicone (GE RTV6708 Sealant Silicone is preferred stuff but 3M is good too) and maybe a new metal strip. The metal strip I don't have a source for but I got mine out of GM's scrap bin at Pole Town last year with a GM suppliers part number on the box but haven't tracked it down or used it yet.
So this is what I have done with another poorly made Bentz sleeper;
First I removed all the trim, which means that I ended up breaking off many of the screws that held on the trim and rusted thanks to the water leaking into the channels of the sleeper frame.
Note to sleeper manufacturers - use stainless steel sheet metal screws, not cheap metal screws that rust. It is a sign of a lazy quality control people.
Then once the trim is off, I had to take a section at a time on each side, removing the gasket from the flange, removing the rusted metal out of the rubber, cleaning both the rubber and the flange with soapy water mix (fels naptha is what I had), then wiping it dry.
Once it was dry, I applied the GE silicone adhisive I had to the flange and to the rubber. Waited the 60 seconds needed to set a bit and then put the rubber back on the flange, pressing the rubber hard against the flange. I cleaned up any spillage (which wasn't much) and waited.
Once I was finished with that part, I waited about an hour for it all to set I then went around the entire inside of the sleeper and cab on the edge of the gasket with a bead of silcone sealent (GE RTV103 Black Silicone). I also applied on the outside the same stuff but before I did that, I cleaned up the suface for it to stick to. I didn't just bead the ourside but I put a bit of a concaved shape to it.
All in all it took me about four hours to complete the work, which included driling new holes for the trim cutting the old rusty screws off so no one gets their hands cut up.
I used five tubes of each, they were small tubes and the cost was about $75, including the new stainless steel screws and a good lunch.
That sleeper hasn't leaked at all since it was done and seeing the truck the other day, after a year and a half it has held up very well.