there's probably only 1 way.
if its like mine, you have to remove the plate over the exhaust/intake ports on the outside. then remove the flange, which is attached to the furnaces exhaust pipe, and holds it tight against the interior wall of the trailer.
then you go under the sink, remove the face plate, disconnect the wires and gas line, remove a couple of screws from the front of the furnace, and it should slide out of its "rack". the outer box stays in place. Its a very tight fit around the galley cabinet, but its do-able.