Try operating the water heater and the cooktop range to get propane moving through the pipes.You may have air bubbles in the gas lines that upsets the ignition in the furnace. When you move the thermostat to a higher temperature setting you should hear a click then a faint whistle inside the furnace housing then hear the gas igniting and burning, maybe a faint smell of propane. There are some 12 volt DC connections and ground connections at the furnace unit that may be loose or corroded causing ignition problems. There is a circuit board that plugs into a connector. Unplug it and clean the contacts with fine sandpaper or pencil eraser. There's more, including a "sail" switch that may be sticking. There have been previous furnace troubleshooting threads. Old furnaces can be quirky.
Try the furnace with engine running. Increased house battery voltage may get furnace to ignite letting you know that there is a low charge level problem with your house battery and/or 12 volt connections to furnace.
