I was watching the other post to see if you would post to it the model number, there is a real reason for the need, some units use a time delay relay which when they burn out will not close the contacts to initiate the fan motor, if no fan then the T.D.R. is suspect, there is a triple redundant feature to the start sequence with a lock out in other models, others use a sail switch to confirm blower...Still others have on the older control board a 3 amp spade fuse which is unknown to most people.
There are known issues with the igniter sensor ground circuit with reference to the control board ground in more modern units.
Without model numbers I cannot access technical bulletins regarding known issues?
The variations to design in furnaces are rather stark, a generic flow chart to repair might help but would not be targeted to your model...The proof that you have 12v with a reference to ground (-12v) is elementary, oft being over looked.
When you say it does not run are we saying its dead except for the thermostat click or the relay within the unit going click? At anytime does the blower sequence come on?
How long did you wait, were tanks recently changed ect.
Its hard enough to guess with a model number, but without one its blind poker, its just a guess based on fairies in the machine.
Starting simple is best, not diving into the complex or exotic issues, ask do I have correct power, propane
, are fuses good, are there any on the board?
Happy Camping, Safe Trails.