Sounds like its not getting fuel (propane).
If your tank is empty or the valve is closed this is the obvious reason why it may not work. We ran out of propane a few weeks ago and it did the same as what you describe, came on and blew air, then shut off after a while.
Might also be the igniter if your propane is getting to the furnace. Listen and see if you can hear it click on.
2009 Jayco X23J
2011 Ford F150 XTR 4x4 Crew
Travel with wife, two boys, food and Canadian beer of course