Most RV furnaces have standard 4" diameter plenum attachments at the side or a slot on the bottom. My 76 28' uses the bottom plenum hole to feed the trailer ducting.
Your rectangular duct, if its like a 78 31' I worked on recently is just an adapter to convert two round furnace outlets to a rectangular vent placed only a couple of inches from the furnce sidewall.
You can get the adapters for the furnace that will allow you to attach 4" flexible ducting ($1/foot at the local RV place here) to the round plenum holes. It is a simple flange thing that fits into the indents on the furnace holes.
You can also get round vent outlets to put on the other end of the vent tubing. They only cost a couple of bucks and come with adjustable air diverters. Put in one for each furnace plenum outlet that is not otherwise used to get good airflow out of the furnace.