Likely debris is blocking the U tube heat exchanger. Spider web etc.
Other is the sliding air mixing control is not allowing enough air and runs rich. The slots open 1/4” is a ball park eyeball. Just less than full open.
To clean heat exchanger. Turn propane off. Remove burner assy. Not the gas solenoid. It’s just a few screws. Remove exhaust vent louvers. Snake a wire thru tube. Attach small rag to wire pull back thru to pull debris. Make sure rag is secure.
2005 Dodge Ram 2500 5.9L 6 Speed
16" Michelins, Hi Spec Wheels, Max Brake, Dexter 4 Piston Disc Brakes, Carslile Actuator, Equal-I-Zer, Dill TPMS. Campfire cook.