The thermostat, if it hasn't been replaced, can be dodgy. But a cheap $20 Walmart 2-wire will do the trick. Before you try it, make sure propane is on, and you have even run a burner on the stove until it lights, which will get the air out of the line.
Sequence for the heater is: fan comes on, a switch will click underneath, then the pilot light, then the gas will light and it will make heat. Turn it off and it will run for a bit on just the fan. If it fails to light, the fan will run for a bit. Easy way to test thermostat is remove the thermostat wires and jump them...if it makes heat, it's the thermostat, if it doesn't there is a propane problem, an igniter problem or a thermocouple problem.