Bill -
The Thetford exploded-view parts list on page 6 of
https://www.thetford.com/wp-content/...c-Model-80.pdf
shows that you are dealing with some kind of circular retaining clip, but provides no details. A little investigation shows that it is what is called a Circlip. Good news - getting the Circlip off is a do-able thing.
If you do a Google search on "circlip", you will get a hit on this page
https://www.google.com/search?client...-1-d&q=circlip
On it you will see that there are two kinds of circlips. One is like a flat washer with a small section of the washer cut out. The other is like a piece of wire bent into a circle, with the ends not quite meeting. Both are made of a springy metal. You are dealing with the wire kind.
Referring to the second picture below, push the lid and seat to the right. On the left end of the left half of the hinge, push the rod to the right. In the gap between the lid and the hinge cover, you can see the end of the rod. If you look closely, you can see what appears to be a little ridge that goes around the rod. This is the circlip, and it rides in a groove in the rod. If you grab the head of the rod (the head looks like a screw head, but no slot) with a pair of pliers, and turn it while you watch the clip, you will see the gap in the wire come into view. Basically, what you have to do is push the two ends of the clip away from you, forcing them to spread apart. When you have pushed them far enough, the clip will pop off.
I posted my pictures in the wrong order, and was too busy to change it. The second picture below is simply to get you oriented. The first picture is the one that matters. The original hi-res photo makes it clearer. If you would like a copy, send me a PM with your email address.
Bill