I recall a long time ago that automotive body shops used to use a small amount of black "plyable" putty of sorts to fix a rubber seal that lost its shape. As long as the seal itself was still flexable, they would apply some of this stuff behind the rubber, to push it outward.
Another old trick my father in law used to do was to apply some brake fluid to a rag, then wipe down a rubber seal. He said it will make the rubber seal swell. I tried it once on a rubber seal used in an old 58 Apache pickup I had with a vacuum driven windshield wiper setup. After applying the brake fluid on that seal, the wipers did work again, as the seal did swell out, sealing it against the vacuum canister.
The bad part of brake fluid though is that it will mare paint, so don't do it if the rubber seal touches a painted surface.
2015 Jayco Jay Featherlight SLX16XRB (TT)
2007 Ford F150 Tow Vehicle (TV)