I realize that you are talking about weather strip seals used around the panes of glass rather than the butyl tape that is used under the window frame when the window is being installed into the opening in the shell.
Many of the older trailers have vinyl types of window seals. They will indeed harden and shrink with age due to UV and the loss of plasticizers.
There are some products around (vinyl softeners) that help with softening those seals again and also protect from UV damage. The one I like to use is "Mothers Back to Black" which is available in many auto parts stores. There are other brands which do the same thing, just talk to the auto parts sales persons. The seals don't have to be black, the product is white colored.
The process I use is I take out the seal and slather on some of the liquid over all the surfaces of the seal, put it into a zip lock bag and let it sit for a day or two to fully absorb the product into the seal. Then wash the product off and put the seal back in place. It has rescued several seals for me including the very stiff ones for the tail lights
on my 1980 Sunrader.
Sometimes the seals have hardened to the point where they are simply too far gone at which point they should be replaced. I was able to find a lot of the replacement seals or at least viable substitutes for my 1970 Campster
so I opted to replace them as they did not respond to the vinyl softener ... too much long term UV damage.
Some of the new versions of the window seals are being made in silicone which has a longer lifespan than the original vinyl products. I know that Vintage Trailer Supply is now having some of their seals made with silicone. So if you can upgrade to any of those then you won't have to do that chore of treating them to prevent damage