Here's a good source for a manual
for your furnace. Should be a troubleshooting guide in there as well.
Sounds like you have 12 volts to the furnace, since the fan runs, but no ignition, so it could be something as simple as a bad or blocked igniter.
If you pull the old furnace out, you might also consider replacing it with a catalytic or blue flame heater; enormous saving of propane and battery power (important if you're boondocking, even overnight).