The fan runs on 12 volts, so if it's running, the converter is passing 12 volts to the furnace.
If you hear the ignitor discharging, then check for a blocked/pinched LP line. Is there a fuel shutoff valve in the line to the furnace that needs to be opened?
If the ignitor sparks but the furnace does not light, you could have a bad solenoid-operated gas valve or a bad control board that's not signaling the gas valve to open. The control boards are a very common culprit.
Eric & Holly + 2 kitties from New Hampshire
1992 Winnebago Adventurer 32'