One of the simplest causes of failure to a furnace is a blown fuse that causes the fan to stop working. Check to see if the circuit breaker is tripped, and identify if the fuse is blown. If so, you can easily replace the fuse. You should always carry spare fuses.
A dead battery is easy to diagnose. Check to see if you have power in the RV. If not, your battery may be low in charge or dead. Without power to the furnace, your furnace won't run.
Bad Propane Tank
If your fan is running but you have no heat, a likely cause is at the propane tank. Check to see if the regulator is working. A bad regulator can stop heat from coming from the furnace. A bad valve on the tank can also eliminate heat from being produced.
If your vents are blocked, partially closed, or obstructed, your furnace can kick off and cause failure to the system. Blocked vents can easily go unnoticed when vacationing and lots of clumsy gear is jostling around in the RV.
If the furnace system is not routinely cleaned, buildup inside the vents and furnace can cause system failure. Maintain a good, clean system and it will run efficiently and strong.