Maybe I'm the last one to figure this one out, but my Thetford toilet in my 07 Excel Limited has been slowly leaking water down. You have to let the pedal snap, up and the valve slam shut like the breach on a .45 acp.
Sometimes this had to be repeated 7 or 8 times to get it to hold water. Nice, at 2:00am ! Thetford sent me new seals after I described the problem to them. Nice people. Any way, there is a plastic shroud around the toilet that is held in place by a pair of industrial rubber bands. This on mine was the problem. The shroud came down just far enough to keep the pedal from fully rising, and letting the slide valve fully seat. I took out my trusty grinder and removed 1/4 in. of material at the top of the notch above the pedal. The toilet has worked perfect ever since. Beats taking the toilet out and disassembling it to put in a new seal. Hope this helps someone else with this same problem.