Collette,
The "termination valve" and the Slide-EZ valve are one in the same. As others have said, pull the valve out as far as it will come (5-6"
before you close the trailer. At the dump site, open the toilet dump valve first (the one on the 4" PVC pipe). After it has drained (helps to have a clear sewer adaptor attached to your sewer hose at the trailer end) close it and open the "gray" water valve (1-1/2" PVC pipe). This will drain your gray tank and clean your sewer pipe, too. Then, disconnect the sewer hose, cap the drain valves and rinse the sewer hose with the water available at the dump site.
The fact that you found green chemical on the floor of the bathroom tells me that you either (1) have a cracked Slide-EZ valve, or (2) the toilet is not sealed properly where it attaches to the floor. When you try to move the toilet side-to-side, does it move (it shouldn't). I had both experiences with mine, and did the repairs myself. The cracked Slide-EZ valve was replaced by my dealer under warranty. I later had to replace it due to the neoprene seal failing and Thetford sent me a new slide ***'y at no cost. If the toilet moves, it does not have a round disk screwed to the floor under 1 of the legs. You need to install that disk and reinstall the toilet making sure you secure the slides under the bolt heads properly. You'll find all the diagrams and instructions in your Thetford owner's manual as well as on Thetford's website.
Best wishes as you solve this messy problem.