Tom's link gives you lots of good stuff to read.
Basically, you need to get the old furnace out, cut down the sides and back of its old cabinet, and fashion the airway from the bottom of the new furnace into the air intake on the top of the old plenum. "Plenum" seems to be just another word for "box with holes in it".
The plenum is what the old furnace sat on, which directed the hot air to the two outlets. One outlet exhausted into the sitting area, and the other into a l-o-n-g duct that went to the very back of the trailer (the bathroom in my TT).
Assuming that your new furnace has the same basic problem (not as tall as the old one) you will have the same job to do: lift it high enough to reach the old intake/exhaust holes in the side of the trailer. I needed to raise the new furnace up 1 1/4" and used mostly strips I cut from the old cabinet we had cut out (see above).
Here is one taken just before we fastened the left rail which was used to give us the extra 1 1/4". The hole in the top of the plenum matched the knockout on the bottom of the new furnace.
Here's another that shows us ready to mount the new furnace:
That's the main strokes. It's harder to describe than do. Just keep reading those links until you 'get' it.
And ask questions when you don't, of course.