Heater wont work

Fan comes on. Moves a "sail" switch with the air. Control board detects the sail switch activated and opens a solenoid valve to let the gas in. Igniter starts sparking. (Can you hear the igniter: "pop, pop, pop"?)

The DSI RV Furnace - sequence of operation.

The thermostat is turned on and calls for heat.
Electrical current is sent from the thermostat to the time delay relay. The relay closes after a delay of 10 to 20 seconds and energizes the blower motor.
The blower motor must come up to 75% of it's rated speed to continue the ignition sequence. If it doesn't - the motor is faulty, voltage is too low, the return inlet air is blocked, or there is a blockage in the exhaust.
Once the motor comes up to speed, the sail switch is physically moved by the air flow and closes a micro-switch to continue the circuit. It must have a certain force to close it and if the motor is not running fast enough, or there is a blockage of some kind, or a "sticky" switch, then the current is interupted.
Note: This circuit is totally independent of the thermostat or motor circuit and only comes into play when the fan blows "hard enough" to close the sail switch.

Locate the two wires that come and go to the sail switch - with your test light, probe each one - you should have power at both wires, with the furnace fan operating. If not, replace the sail switch.

The sail switch closes and sends the power to the limit switch. The limit switch is normally closed, but will open at a preset temperature to prevent furnace overheating. With the fan running, probe each terminal of the limit switch - you should have power at each terminal. If not replace the limit switch.
The limit switch then sends power to the circuit board - the circuit board delays ignition for 10 to 20 seconds to allow the blower to purge any gas from the combustion chamber.
After this delay, the circuit board opens the gas valve and sends a high voltage current to the electrode (igniter). This ignition TRIAL will last for about 10 seconds. If ignition occurs, the electrode senses the flame and returns this information to the circuit board. The circuit board then stops the high voltage spark but keeps the gas valve open. If ignition fails, the board will go into lockout (new boards will try three times before going to lockout).
Lockout can be caused by a faulty circuit board, faulty gas valve, a faulty electrode or any of the above conditions.
If the ignition sequence is interupted at any point, there will be no ignition. Safety is utmost!