I do not believe that there is any such animal.  I have looked and looked.  The only 12v devices are not tru a/cs, but rather are evaporative coolers.  There is one ac that is pretty interesting -- it is the starcool which I believe is only available as an installed option from sportsmobile (sportsmobile.com).  It is not the roof type, but sits inside the van -- not sure it would work n a westy.  It operates off the motor and ac power.  Some of the earlier stuff I saw on it I thought suggested it was 12v but the more recent stuff clearly says it is 120 and motor driven.