Greetings Glyn!
I suspect that you will find a situation with your Caravel similar to
that I had with my Minuet. The coach will likely be pre-wired, and you
will also find that the condensate drain tube(s) have been
pre-installed; but I suspect that there won't be a drain pan as each was
unique to a particular model air conditioner.
With my Minuet, the opening was cut as measured from the front main roof
bow - - it basically encompassed the electric box cover that was visible
in the ceiling just ahead of the rear roof vent - - I don't think that
Airstream used the same method as Argosy to identify the Air Conditioner
pre-wire; in fact I am beginning to think that the ceiling mounted
electrical box cover may have been a Minuet only feature where the
pre-wire was an extra-cost option according to the literature that I
have.
You might try contacting the factor to get the necessary measurement and
the point from which the measurement needs to be made.
Good luck with your project!
Kevin
Kevin D. Allen
1964 Overlander International/1999 GMC K2500 Suburban
1978 Argosy Minuet
1975 Cadillac Convertible (towcar in-training)
WBCCI/VAC #6359