I have a Dometic-Sealand Concerto with a ball seal, and I too have found Vaseline to work... for about a day or two.
Then I use a heavy dielectric grease (used for electrical contacts) and it lasts 2weeks to 2 months or longer!
...And occasionally I have to dip my hands into the water and push the ball forward to seal tighter. I think the rubber gets hard over time and/or dries out during storage and you have to reform the seal by adding some brut force. ...In my case I can feel about 1/32" of play.
So try using dielectric grease on the seal side and see if that fixes your problem.