This link identifies the items you're looking for. Unfortunately it doesn't solve your problem completely. The diagram identifies the vent plate, vent hose and seal are what you're looking for. I've not seen the vent plate for sale anywhere, but the hose and seal are out there. I'd reach out to Thetford directly to see if the plate is available as a separate item to purchase. If not you may have to purchase from an overseas store if its even available.
When I bought my first Thetford cassette toilet, it didn't come with installation instructions. Seems the US division didn't have a copy to provide. (Which seems really dumb) Once I received a can't help response from the US folks, I forwarded that email to the European division and they sent me a complete useful copy. The europeans didn't want to help, until I showed that the US folks couldn't help me. Hope this helps you.
The last option for the vent plate is if it can be made/copied, with a 3D printer.
https://www.caravansplus.com.au/spar...t-p-11810.html