So , what year trailer is it? What model furnace?
On my '69 Overlander, it is fairly simple. It has a Suburban NT 22 furnace. Shut off gas supply. Shut off power to it , should be only 12 volt
& turn thermostat off. Disconnect the gas at the unit. Remove the vent from the outside. Remove the Front of the furnace by unclasping the latch at the top, pull back towards you and down. Front door comes off. Then there was a screw/bolt in the lower right corner. Remove it and then the inside pulls out from the case. That leaves the case and duct work hooked up. Don't need the case out to repair it.
Good luck, Loren