Most people probably already know this but for the few that may not.

My Thetford toilet bulb stopped closing completely which of course means that water would not hold in the commode.

I called Thetford to order a new spring assembly and they suggested that I purchase silicone grease and apply it to the rubber seal.

I cut off the water to the RV -- wiped off the rubber gasket and applied a liberal amount of the silicone grease to the top and bottom of the rubber seal and some on the top of the bulb -- problem solved.

Probably will do this every few months or whenever the bulb starts to get stick -- whichever comes first.