Some time ago on another thread, a person suggested olive oil in the toilet when winterizing and I tried it the last two years. The olive oil does not evaporate like the antifreeze or water do so it would seem to me that the seal will be covered. It does look a little flunky when you empty it in the spring, but a little wipe with a cloth will clean it up nicely. And, the olive oil certainly will not hurt the black tank when it gets there.
2012 GMC 2500HD D/A Crewcab
2012 Jayco Eagle SuperLite HT 26.5 RLS
2009 Crestliner Superhawk 1900
2013 Polaris Sportsman 550 EXP
2008 Polaris Sportsman 700 X2