I think he means the gate valve that you open when you dump the recirculating toilet, except that he thinks it is going into another holding tank which the TM doesn't have. I could be wrong though.
If it is the gate valve that you open and close at the bottom of the toilet, the part number is 12836 and appears to be currently available on Amazon:
https://www.amazon.com/Thetford-1283.../dp/B003D5BOL8