Don't rush into removing the furnace quite yet. There are some others things to check first. Does your propane stove work in high flame? If not, the main gas valve may be shut, the propane line and/or regulator may be full of debris, or the propane sensor may have shut the system down for safety. Check those first. After you remove the outside panel of the furnace, you should be able to see the burner tube, the gas orifice, and the ignitor. If you don't see a spark but have 12V at the board, chances are the board is bad. You can disconnect the high voltage wire from the board and, with an insulated wire screwed to a good ground, hold it about 1/8" away from the top of the high voltage coil and see if there's any spark there. A voltmeter is almost useless 'cause it can't read a short duration, 10,000 volt pulse very well. Besides, it can fry your meter
If you see a spark that way, chances are good that the gap between the end of the ignitor (near the end of the burner tube) is too large (re-position it according to mfg's instructions) or it may be shorted out by the presence of carbon from a dirty (yellow) flame. It should be blue. The ignitor can be removed and cleaned with a rag and some solvent. Next thing to check would be the gas valves themselves. They're actually electrically operated solenoids, and there are usually two of them (for sfaety reasons)mounted together. When you turn on the furnace, you should hear a click from them, bt it may be difficult to tell if both are operating (as they must). If not, check for 12 volts on their terminals. No 12 volts? Check wiring back to circuit board, check the inline diode with your meter set to ohms (it will read a resistance when you have the black lead on the cathode (the end with the black band) and the red lead on the anode. When you reverse the two meter leads, it should read infinity. If it reads infinity either way, the diode is bad. Also check the limit switch with the meter. It should read almost zero ohms. Check the solenoids for continuity. They will probably read somewhere between 100 and 1000 ohms if o.k..After all this, you can further check the solenoids, but be very careful!: Turn off the propane at the tank.
Remove both wires from one of the solenoids. Connect a ground wire to one of the terminals (doesn't matter which one) and with another wire connected to 12 volts thru a 5 or 10 amp fuse, briefly touch it to the other terminal several times. A definite click should be heard each time. Reconnect that solenoid and repeat the process on the other one. If either one fails to give you that click, it's bad (gummed up or corroded) and needs replacing. You should be able to do all these things without removing the furnace, but you didn't tell us which make and model you have. That would be most helpful.
A word about ignitor boards: Those for water heaters typically send out one spark to light the flame; those for furnaces send out multiple sparks. Sometimes a furnace board will not do so, and the furnace may not light. If in doubt, most reputable RV repair shops have board testers and will check them for you. If you need a replacement and you have an Atwood or Suburban furnace, I would suggest replacing it with a Dinosaur board.http://www.dinosaurelectronics.com/