Hey.
Don't know about a Van.
But yrs ago we had a 12ft6in rear door Caravan.

It had a fold out unit that fitted along rear of van. Above doorway.
Looked like the modern roll outs we have on utes.
but along rear of vehicle.

It rolled out 8ft. width of van (7ft?)
had fold out sections in there to give rigid frame round top and 2 end legs telescoping.
Then guy ropes for stability

Velcro end and 2 sides then glued on (with door on one side)
to make a full extension room 8x7 off end of van.
???? Maybe.....
Was made of lighter annexe material and I've seen some 10 yrs and older. still good.
Took me around 5 min's max to erect.

Check around some of Annexe mfg's or older Established,van mfg's too.
For contact no's

We used that van in Darwin in the '90's for 3 months at time. gave a 20 ft van
with 10ft roll out alongside of van for shade
and a second hand House aircond sitting on blue plastic 44, to one window.
Beer carton cut to size for airflow.

Just like home. But a little small for travelling in.