By the looks of an exterior view there is just a air in/exhaust out grill so it would be a Suburban Brand furnace.
It has to be accessed from inside the rig........and may have to be removed from cabinet to work on.....depends on model.
When you have thermostat set to heat and fan in auto.........what happens when you call for heat (run set point higher than room temp)?
Does furnace fan come on?
If so..........does it try to ignite the main flame? (can you hear the spark 'clicking')?
What is it doing OR not doing?
The furnace runs completely off 12V DC power (fan/gas valve/controls) so you have to have good 12V DC power source
And the furnace needs good supply of propane.
Post back some info.....it may be something simple.........if not Mobile RV Repair