I just did ours. Cleaned the seal with a Velcro strip rubbing it on the seal. I then rubbed some toilet valve conditioner on the seal and then inspected the seal with a small inspection mirror on a rod to make sure the seal was clean and in good condition, closed the valve and put some additional valve conditioner in the toilet bowl and let it sit for several days. It now holds water perfectly.
I also have some of the plumbers grease mentioned above on hand in the RV for future use if needed. Also make sure the slide valve or ball closer depending on which you have is clean and smooth.