Question

Have a 1973 camping trailer with a Coleman 14000 BTU furnace, Model 9215. It has no blower unit---just comes on and passively heats up. Manual pilot light. No apparent electronics, or wall thermostat. However, there is a small black temp control knob (low to high markings) next to the red knob on the main gas valve. Connected into this valve is something that looks like something similar to the thermocouple next to the pilot light, but looks like a fat, bare copper thermometer; the thing is clamped close to the exterior of the burner housing (to possibly detect temperature??). the metal "wire" at the end of this fat thermometer goes into the gas valve box, near the temp control knob. Problem----the furnace comes on o.k., draws outside air and vents O.K., but the temp control doesn't seem to work---it ran continuously for 30 min, and got quite warm, and so I shut the thing down. Any help/advice. Any generic part help? THANKS!