Also, in the time the unit turns on, do you hear it light off? If it does, then shuts down, that could be the system's temp limit switch.
If it doesn't light off, I would bet it's propane (or lack there of). This past weekend, we had some friends come visit us with their Heartland product....we had to cycle the furnace by letting it run and try lighting to purge the system. It took about five tries to get the system to ignite.
TV: 2015 F-350 DRW, CC, 4x4, with 14k GVWR package, 98 gallon TransferFlow Aux Tank, Timbren Shocks
TH: 2014 Voltage 3950, Sailun S637 all around