Furnace won't ignite

It was the # 4. Three flashes at a time. So what does that mean?? Plenty of propane as both tanks are full and the stove and oven both work.

Check to make sure you don't have a faulty regulator on the propane tank. If that's okay, try lighting your stove top burners (all of them), to get the air out of the lines. We used to have to do that to get our oven to light.

Also check your battery levels (assuming you are not connected to shore power). In my old trailer if my batteries started getting close to 50% I didn't have enough juice for the igniter to kick on. If you are connected to power, obviously it wouldn't matter.

The 'little red light' blinking is a 'fault' light.
How many times/how fast it blinks indicates what the error is.

Fault Codes:
1. Internal Circuit Board Failure (Steady on, no flashing)
2. Limit switch/Airflow problems (1 flash with 3-second pause)
3. Flame Sense Fault (2 flashes with 3-second pause)
4. Ignition Lockout Fault (3 flashes with 3-second pause)

SO.....which one is 'problem'?

Not to insult your intelligence, but are you sure you haven't run out of propane or the bottle is not turned on? The selector switch between your two bottles might also be in the wrong position. It's happened to me before...

Another possibility is that there is air in the gas lines. Try running the stove for a couple of minutes to make sure you have good flow.