For about 3-4 years now we've been dealing with the Thetford toilets loosing water in the bowel. There have been countless fixes and repairs that have only proved to only last a short while.
Don't waste your time going to Keystone, Thetford or your RV dealer. Getting a replacement or replacement parts IS NOT going to fix the issue! Your solution is to buy a Dometic 310 or 320 model or attempt this permanent repair yourself. Here's why....
Below is a photo showing the issue. As you can see the plastic flap will not fully cover the gasket. The plastic flap is barely touching the gasket. That's why adding grease or letting the foot pedal slam shut, allows the flap to come in contact just enough to hold water.
You can see the bottom of the plastic flap still has more room to move across the gasket to get give better seal. And even then there is not much plastic material to cover the whole gasket.
Below is where the permanent fix comes in. Within the circle highlight you will see the white plastic which is part of the foot pedal that stops on the triangle tan colored plastic. That tan piece is part of the molded base of the toilet that attaches to the floor.
Below, I hope is a closer view.
Below, that tan plastic needs to be shaved or ground down using a Dremmel tool with a sanding disk or what I used was a very sharp utility knife. The knife is all I had with me during the time.
You only need to remove about a 1/16"-1/8". This will allow the foot pedal to move up further thus allowing the flapper to further cover the gasket making a permanent seal and fix.
To start the repair, have someone hold the foot pedal down while you cut or grind away.
The great part about this repair is that you DO NOT have to remove the toilet or stick your hands into anything.
I've done this repair almost 3 years ago and have never lost any water even after months of sitting on my property.
Below are photos showing the Dometic design. As you can see it seals much better than the Thetford and has a more robust setup.
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