It's a black, soft rubber elbow that kind of looks like an accordion where it flexes at the joint.
My manual doesn't have a parts diagram. I'm actually about to put mine up in the driveway and do some work so I will take a pic at least.
Found this online for C400 series toilets but its i Australia:
https://www.caravansplus.com.au/5180...s-p-13790.html