Had 3 Thetford toilets in the last 11 years of FTing. You have 2 possibilities of your issue. The vacuum break and the water valve. All of mine were water valves. I keep one in stock at all times. The valve leaks out the shaft. They are not repairable. Must replace. Leaks only when the toilet is being flushed. Use a flashlight and maybe a mirror to see the water dripping off the linkage. Remove the toilet, replace the valve and all is good again. Make sure you take a pic of the setup (linkage-return spring) after you remove the toilet. Reinstall exactly the same way. About a 15 minute job. Eazy-Peazy. rockin'