The intake/exhaust port should be removed first. Take out the screws and it will pull off with some effort. It has two tubes that slide into matching furnace tubes with a very tight friction fit. Once you manage to get it pulled off, the furnace will be free (except for any screws that hold it to the floor). Often you cannot pull the furnace far enough inward before hitting something to free it from the plate pipes. I was thrilled on our RV that the outer port plate was very large. Removing it lets me pull the furnace out that way!
