When I got my 1997 sportsmobile in 2008, my suburban furnace would not work. Took it to RV place and they blew a ton of dust out of the furnace by blowing into one of the side vents. Furnace started right up. I think they had seen this problem many times probably due to dust on the igniter. Been working ever since.
Some times if furnace does not light, I light the stove and that seems to get the gas flowing for some reason.