I had the same problem with my Thetford Electra Magic RV80 model. The product ID label is on the front of the main housing. The seal in the Slide-EZ valve became defective and would not allowed the slide to close. I replace the entire slide valve. However, the following year, the seal in the new unit became defective. I found out that you can order just the seals and rebuilt the valve. This was at a great savings and the job was simple. That was last year and it is still working. If you do not have the information that came with the trailer, the parts lists and installation instructions are on the Thetford web site at
www.thetford.com. The installation was ok, but you may have to make your own tool to pull out the slide in the brackets that holds the toilet and keeps it from moving on the floor. A little trick is to remove the cover/bowl assy so that you can align the main housing to the drain. I just replace the cover/bowl assy due to two cracks in the cover. Still cheaper than buying a new unit. You also can call their service/parts info at 1 800-521-3032.
Hopes this helps. Ralph