Actually it is easier than you think.
The gasket (its real name) uses what I call finger stock (a steel strip with sharp protruding fingers) to keep the gasket tight and sealed against the frame. This allows the gasket to flex a bit but not move off the frame.
The finger stock is made of cheap metal, when it should be made of stainless steel. It rusts and bubbles under the rubber gasket, leaving holes or gaps to allow water to move into the sleeper.
I repaired a lot of these, most were from Bentz, but did a couple other manufacturers.
Here is what I do.
I strip the plastic (or wood) coverings off on both sides (you don't have to but it sometimes good to get both sides done.
I carefully peel back the gasket lip starting on one side and work my way around removing the rusted finger stock. Just doing a section at a time (usually 18 inches).
I wash the section and dry it with a rag or paper towel.
I then apply GE Silicone Sealant II - the flexible stuff to the frame and to the gasket surface - only a small amount like a 1/4 inch bead. I wait for about 30 to 45 seconds and then apply the gasket to that section.
I didn't do this for a long time but I have started again to put binder clips (those LARGE ones) on the gasket if it moves. I "soften" them a bit by prying them open but you can use cloths pins.
I then move on to the next section working my way around.
I don't always remove the finger stock if it is good, meaning not rusty, but I do use the silicone to seal it.
On some sleepers I also do the outside, I found a few of them seem to have problems with the frames so I run a small bead under the front of the frame under the gasket while lifting the gasket lip up.
SO far all the ones I worked on have yet to leak.
By the way, I also used that silicone to fix the rip in my sleeper, holding up so far.
I would also check the panels for water damage. These sleepers having leaks will allow mold to form, which I think the manufacturer should be the one to repair it because it is a serious health hazard. One sleeper was gutted for me to fix had black mold and the owner had a company come in and actually do the cleaning. He wasn't going to put his people into that sleeper for any reason with mold growing. Many manufacturers use cheap material and don't actually engineer their product right.
Oh I was talking to someone about gasket issues the other day, and they clean theirs with soap and water after every truck wash. So I have to ask, how many of us actually clean the gasket with mild soap and water and then hit it with a little Armor All?