It requires some fabrication, unfortunately.
Once the old furnace is removed, you need to remove the furnace housing from the trailer and then seperate the bottom piece from the furnace housing.
The part you need is the part with the 2 outlets on it, it will end up being a flat box with a hole in the front.
It comes off by chiseling the spot welds. Yes, sharp cold chisel and mallet.
The new furnace then goes on top of the old duct connector, and you will have to fabricate a cover for the front.
It is entirely possible that you also will have to make new holes through the skin for the furnace vent/exhaust.
The burner assembly comes out with the housing still in the trailer. It is easier to remove it as 2 pieces.
If you remove the living room heater vent, then it is easier to pull out the box.
I had never used the furnace in 2 years, so i just removed the defective unit, but saved the plenum box and installed it without a furnace. This way I can always put a furnace back in if necessary. The catalytic heater is usually enough for camping where we like to camp.
There is a factory bulletin on how to replace the furnace. PM me and I will fax it to you if you want. ( Hope I can find it...)