This Thetford product might help.
http://goo.gl/9q8fG3
When mine wouldn't hold water, I got a cheap, firm, toilet brush from Walmart.
Stepped on the flush lever and gave everything in the flap, seal area a really good scrubbing . UP and down while twisting the brush, after the third time doing this it stopped leaking.
I guess toilet paper gets caught in the area of the seal.
I thought when I retired my Hobby would be RVing.
I was wrong, my Hobby apparently is Finding and stopping leaks.