I have an installation where I have a Vanagon a/c ducting in the front cab
ceiling.
I have a removable duct that goes from one big vent in the rear a/c to the
front duct which starts just ahead of the pup-up roof.
Its made of plastic tubing, about 3 x 6 ins. in section.    It comes off in 5
secs and stores in the top bunk area after its raised.
Here in the sweaty South East I cant get by with the rear mounted a/c.

It works really well except that now I turn it to low after a while and then
the compressor whines.     Before it was always on high and I couldnt hear
anything much with it on, let alone the compressor.

Steve Cotsford