Could be a number of things, but sounds like your inlet is clogged which tells the safety flame detector to turn off the flame. The inlet has the air filter in it.. Can you trace the inlet from the furnace back into the camper to find the inlet?. If cleaning the inlet doesn't work, clean the flame detector rod. It keeps the furnace from overheating but if dirty it turns off the flame but some gas still goes up the exhaust. Its easy to clean on most furnaces. Turn off the power. Look inside for a rod that is located in front of the burners. There's usually one bolt that holds it on. Take it out and clean it with emery cloth or brillo pad. If that doesn't work, could be the control board, or at worst a cracked manifold. Hope this helps.
