Why no continuous furnace fan option?

I run my Suburban Furnace going down the road anytime I am cold. The dash system will not supply enough heat to keep us warm. Though I have found out that if the furnace flame goes out, the fan will keep running blowing cold air, so maybe you could just shut off the propane and start the furnace, the flame will not light and the furnace will go into the fault mode and the fan should keep running. At that point you can turn your propane back on and the furnace will not start up though the fan will still be running. When you get to where you need the furnace and the propane is on, just shut off the furnace at the thermostat for a minute or so until the fan quits running and then turn the furnace back on. I found this out when we woke up freezing one morning where the furnace had gone out during the night and the fan had not shut off, it was just blowing cold air. I pulled the furnace fuse and then restored power and the furnace took right off and started supplying heat. I later discovered all I had to do was shut it off at the thermostat and it would restart when the thermostat was turned back on.