hscole If you look on the outside of the coach below the sink window you will see the outside of the furnace. You have to remove the access plate. The entire plate not the exhaust/intake port, that will slide out of the furnace. Four screws hold the plate on, Break the caulk that is between the plate and pull it straight out the small ports will slide out with it. All furnace service is done this way. If you have the manuals that came originally with the coach, you should have the suburban manual. It will show an exploded view of the furnace. If you can't find the manual try here.
To pull the furnace out, you have to disconnect the wiring and the propane. Make sure your propane is shut off. Over the years I have tuned mine up and have had to replace the blower motor. Painful process, but I like to do my own work. FYI, I tried removing the "Vent Cap" and found it will just slide out of the furnace. When I attempted to remove it was so solidly caulked to the door, that I ended up bending the vent cap plate severely. That is why I recommend against pulling it first.