David, you are correct that Monty's caravan and associated trucks are preserved at the IWM Duford site, however they are arranged for 'display' rather than photography.
There is access via a raised walkway, which allows a look inside, but completely covers the one side as far as taking photos is considered. The other side is not particularly visible either as there is a diorama, so you cannot get very close, see below
The prospects of photography are not improved by the gloom in the 'Land Warfare Hall'.
I hope this helps.