G'day Richard,
Interesting to know that there was a Coventry Knight floating around (metaphorically speaking) Queensland at some stage in the past.
Coventry Knights were built by the Coventry Steel company and designed by Clifford Dawtrey. They were a BIG unit in their time, with multiple rooms, central heating, baths, etc, etc. Looked as much like a small railway carriage as a caravan, at least compared to Australian caravans of the same era. There's a fair bit of info on them in the first volume of Andrew Jenkinson's books on UK caravans (the volume covering 1919-1959). The early Coventry Steel caravans were clad in -wait for it - steel, but Dawtrey was a bit of an innovator and by 1948 was building caravans using aluminium and perspex. Coventry Steel also were into exporting their vans, so I guess that's where the van you've referred to comes in?
The description of the van in the advert as the Rolls Royce of caravans may have been a bit of hyperbole on the part of the seller. I've seen other British vans described the same way. However, according to Jenkinson the Coventry Knights were amongst the most expensive vans around in 1948.
Regarding Duralumin, a number of Australian Viscount caravan models in the 60's were called Duralvans, apparently because they had frames built out of Duralumin. There's a bit of discussion about it
here. According to an entry in Wikepedia,
Duralumin is "is the trade name of one of the earliest types of age-hardenable aluminium alloys".
Don Ricardo