Assuming you have an original Suburban NT 30 furnace the 1972 service manual (no service manuals prior to 72) shows an 'adapter vent cap" for the exhaust which the combustion chamber connects by a exhaust tube. Does not appear to show a intake tube just the connection to the intake inlet at the rear of the furnace. As I recall where the back of the furnace connects with the skin of the trailer is sealed with caulk or Vulkem.
But the question is do you want a 48 year old furnace in your coach? I, and others, have replaced the original with a new Suburban. IF you have a NT22, that model was recalled ages ago because of CO leaks. I think the replacement model is NT 30S or NT 35S. A little modification is necessary. but a much better furnace.
Try searching furnace replacement.
You will also be hard pressed to find parts or anyone willing to accept the liability of working on your antique furnace.
Sail on silver girl. Sail on by. Your time has come to shine.