This is why I've always used a little bottle of "toilet seal" preservative. Without checking my RV at the moment (in storage), I believe I buy Thetford brand at camping world or Wally World.
I use it year round with just a splash of water. It keeps the seal moist during storage and it never gets stuck from sitting dry in hot climates when your not using the trailer.
It's designed for toilet seals and will not crack the rubber seal. I'm not sure, but I believe it is NOT petroleum based. Many rubber products crack with a petroleum based oil or grease. Not sure what your rubber gasket can handle, so the solution specifically made for toilet seals is probably better for most than Vasoline or grease.
Silverado 2500HD Duramax DIESEL/4x4/CC/Z71:W
31' Travel Trailer
2 Honda EU2000i's (parallel)
ICOM 7000
Random wire, dipoles, verticals
Hunting/Fishing gear
Welding/Fabricator
Life endowment member NRA
:B :B