It might help if you shared which model Voltage you have. On most, the thermostat has to be cycled from A/C to heat. Press the button until you get to heat, set the desired temperature. The furnaces in these units are forced Air propane, so make sure you have propane, and it's turned on. If the propane has been off, or not used in awhile, you may need to purge the lines of any air first. I usually light all the burners on the stove to make sure propane is flowing.
Depending on how cold it gets, you may burn through propane pretty quickly. Make sure your heat vents are not blocked, check the one in the garage. If you have adjustable vents, make sure they are open. If you have an electric fireplace, that can provide additional heat. We also use one of those small cerimac heaters for supplemental heat.
2013 V 3105
2016 F350 King Ranch CC 6.7 diesel dually