We've owned a motorhome since 1986 and don't ever recall lighting a furnace. In the old days we had to light a pilot on the hot water heater but never on a furnace. Have you found the furnace off/on switch and thermostat? You should be able to just turn the switch to "on", the blower should start up in a few seconds and the burner fire up in a few more seconds - - assuming the thermostat is calling for heat and the propane is turned on.
2006 Jayco Greyhawk 27DS
2019 Ford Fusion Energi PHEV