You do have a 2nd furnace, same size as the front one. You'll find it behind the exterior venting panel on the outside of your RV. You'll have another one for the front furnace and also have one for the water heater. (I'm talking about the outside grills/vents).
Once you have the cover off (6 screws usually), check that someone hasn't turned off the gas supply to that furnace. Check for obvious problems like broken or loose wires on terminals.
Most common cause of your problem is a bad 'sail switch'. This is a small micro switch with a long lever with a small metal tab on it. The air flow causes the switch to close and turns on the gas and spark system.