[jsb5717]

Looks like your furnace is, as you said, under the upper floor but with no exterior access panel, only the single exhaust vent. By its placement on the door side of the trailer it looks to me that it would be fairly easily accessed by removing the wall in the basement. You still might need to remove the furnace to work on it, which makes troubleshooting more difficult. Definitely slower but will still work.

Before diving in to it can you explain, in order of events, exactly what it's doing and at what point it fails.

Ex: you adjust the thermostat to call for the furnace to provide heat. then:

1. Does the fan kick on?
2. Does the fan kick on and then attempt to light?
3. Does it light and then turn back off?
etc.