I believe that if moisture got into the propane, the valve may ice up.
When the thermostat calls for heat, a relay fires, supplying power to the furnace control board. The blower motor starts up and if it moves enough air, the sail switch activates. With the sail switch activated, it'll turn on gas flow and try to ignite, and then checks for successful ignition.
If you hear the relay click, and the blower starts pushing cold air through the ducts, after a few seconds you should hear the "whoosh" sound of the burner lighting. A few seconds after that you should be able to feel warm air coming out of the ducts.
A common problem is poor propane flow. On fifth wheels, if the high-pressure regulator on the doorside tank starts to fail, or the high-pressure pigtail on that side starts to fail, you may have reduced propane flow. Also, if the tank valve was opened too quickly, the built-in protection device may shut off propane flow. If you close and slowly reopen the tank valve the problem might be resolved. If that doesn't do it, try swapping the tank to the other position to eliminate a pigtail/regulator problem.
Another problem is something blocking airflow, which would keep the sail switch from activating. Make sure there's nothing blocking the air return and check for insect nests at the back of the furnace.
Be sure you have plenty of power. If you aren't on shorepower, your batteries may be too low to power the ignitor.
