Another thought. If you are going to coat the sealing surface with plumbers grease, first, clean the bottom of the seal (the sealing surface) as well as you can. I usually use a wet paper towel with Dawn.
Oh, and welcome to the forum!!
Scott and Liz - Southern NM
2012 Wildcat Sterling 32RL - w/level up (best option ever)
2007 Chevy 2500HD 4dr short bed Duramax w/allison
Reese Fifth Airborne air ride king pin coupler with Sidewinder