That doesn't sound like any Carawagon rear tent I've seen.
Normally there is a self-supporting frame which pulls out from the rear of the roof section. Note there are no uprights - the frame poles are cantilevered, with a couple of foot left inside the roof when fully extended.
Here you can see the end of the frame (just above the rear door) when pushed home:
Here you can see the bars extended, with a sun shade (pink cotton bed sheet!) spread over it.
As you can see, no uprights needed

:

The Dormobile rear tent on the left (Mick's, Jabbawocky on here) is a custom made awning with a frame and 'hooks' on to the back of his Dormobile.
This picture shows an original Carawagon rear tent on the right, and the roof frames extended on the two Carawagons behind and a tarp thrown over and down the back - our usual 'show' configuration:
I think if you've got some sort of frame which needs putting together, it could possibly be an after-market one, possibly a small caravan awning
Cheers,
Robin.