I just took ours out to replace the skin panel:
http://vintageairstream.com/Myrtle/axle.html
The intake and exhaust pipes you noted are slip jointed together midway
between the skin and the furnace back, and then sealed circumferentially
with a piece of duct tape. Mine was able to twist/pull apart, but you
may have to soak the inside joint with penetrant, accessed from inside
the burner can and the outside cover.
There used to be an install manual on the VAC website manual section
that should the installation.
http://airstream.org./resources.html
Toby, where'd the manual section go?
Best of luck,
RJ