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Hi my name is Robert...
This could be a number of items causing it not to light off. IF it is SPARKING and not lighting off it could be a LP pressure issue, bad electrode or circuit board. I think the key is that you were able to bleed the propane line and it worked for some time. SO it would be very hard for you to check the pressure without a manometer, most RV shops have that tool and it would be the very first thing I would check for this symptom, if the pressure was off then the furnace might not want to light off. GO to the main tank and shut the valve, SLOWLY open it back up, sometimes if that valve is opened to fast the regulator can lock up and you might not get the proper pressure, beyond that I would have that part checked.
Also it could be the electrode of circuit board, the electrode will sense the presence of the flame and the board will shut it down if it does not sense the flame, a RV shop can also test out the board with a special tester to see if it is a fault, some charge a small fee and others will test them for free.
It really is a process of elimination and the key is the LP issue and bleeding it off. Tells me you might have a LP problem.
IF you have more questions or concerns please feel free to contact me... I am around daily and respond when I can, follow ups are included as well. Please keep in mind I only get credit when you select a rating at the top of the page.... Robert