Simple things first. Are you sure you have gas? Turn the range on to check that. Also check that the gas valve for the furnace is on. Next would be to be sure the battery voltage is high enough. Is the rig plugged into shore power so you know the battery is charged and the converter/charger is working? Low voltage can cause the furnace to not ignite.
From there, it is stuff like mud dauber nests in the intake/exhaust, the electric gas valve not opening (rare for a 2 year old furnace) and of course a bad ignition module or contacts where things plug into the module.
Those are some places to start anyway.