Not all furnaces use a sparker that you can hear. Suburban furnaces like on our Casita
have a glow thingy that does not make any noise.
On our furnace
when the thermostat calls for heat the fan comes on. The air moving causes a "sail switch" to activate and shortly you'll hear the gas solenoid click once.
Now the fan runs on 12 volts so if your 12 volts are low or the motor is not turning fast enough the sail switch will not activate that turns on the gas.
On your propane tanks, do you have a green plastic know knob or the older brass fitting?
Don't assume the lady that fills the propane tanks knows what they are doing!!
You do know what "assume" means I hope. Anyway the threads are backwards on the older tanks and you'll need a decent wrench to remove them.
The newer ones with the big green knobs have right handed threads (normal) that can be taken off by hand but you may need a glove.