RV Repair: heater wont work, flame ionization, rv air

Question
2000 bounder fleetwood..
uses Lp ,every appliance in rv works including hot water..when turning on heater to heat rv ,air blowing from vent remains cold...I have to turn on engine to heat rv

Why air from heating unit not getting warm?

Answer
Hello Tanner,
The first step in diagnosing an appliance is knowing the order of operation:
First, 12VDC(Volts DC) must be present to the Furnace's circuit board.
The board will then send 12VDC to the thermostat. The Tstat acts as a switch - on or off - when the Tstat is cooled below set temp, the switch is 'closed' or 'on'. This completes the circuit and returns 12VDC back to the board.
The board senses the Tstat circuit and begins to run the fan(sometimes there is an overload protection on the fan)
(since your fan is running, you'll begin your diagnosis here)
When the fan reaches speed, the wind created pushes on a small flap and closes another switch called the "sail switch".
This sends 12VDC to another switch called the "limit switch". This is an overheating prevention device that breaks the circuit if the furnace gets too hot.
If the circuit reaches this point, it's sent back to the board which tells the gas valve to open and igniter to spark.
When the flame lights, the board senses the flame through a process called "flame ionization". The flame actually acts as a "wire" to complete a circuit from the tip of the spark probe(the small metal rod encased in ceramic) to the tip of the ground probe.
When the board senses this circuit, it tells the gas valve to stay open and the igniter to stop sparking.
The furnace runs until the Tstat warms up enough to 'open' and break the circuit.
The board senses this, closes the gas valve, runs the fan for 30-60 secs and then waits for the Tstat to tell it start all over again.

Phew... Now, for your problem, you need to determine where in this order are you getting lost. If the valve opens, you'll here an audible "click" (sometimes it's easier to put your hand on the furnace and feel for it. If it sparks, you'll here "click,click,click..." really fast. Are you getting either of these to happen?