You may have one of two problems or maybe a combination of both.

Check your air inlet and exhaust ducts for proper airflow. Make sure no bugs have made a home inside them, and that they are clean and open.

Check the air flow of the blower motor for a good volume. If it doesn't maintain a majority of its rpm by low voltage, worn bearings or a dirty impeller, it won't provide proper combustion when the sail switch opens and fires the ignition circuit.

In addition, you can also check:

Low gas pressure which can also cause incomplete combustion. Check the ignitor to see if there are any bug's nests affecting the burner's operation. The flame should be sharp and distinct, blue in colour with little orange surrounding it.

Many problems are caused by dirt or bugs' remnants. A good cleaning may all be that is necessary, to return the furnace to a proper running condition.

Gard,

Thanks for your help. There were a couple of mud dauber nests in the exhaust tube. Hopefully this will correct our problem.

You mentioned a blue flame, but I can't see any flame. Is there a shield, or something, that needs to be removed in order to see the burner?

Thanks again for your help. Your advice is always accurate and very helpful.

The combustion area of a furnace is in a contained space, to prevent the chance of creating a fire to any nearby structures. It is usually accessed by a moveable/removeable plate or cover. Finding this is usually an easy process. The propane gas line and wiring from the circuit board have to meet at some point. This will be in the burner section of the furnace.

Some time spent on line should produce a diagram that would indicate the component(s) you are seeking. You can enter your furnace's manufacturer and model number into a store's data base, and a catalog and diagram of parts will be presented.

There you will find an ignitor and a gas orifice. This is where the flame is created. Any dirt here will cause the flame to be less than optionable. There will be a nearby gas/air adjustment in the gas line, that will allow the flame to be manipulated to its most efficient setting.

Your heater is working, so the burner section is presently operational, perhaps not at an optium level. The fact that it takes a while to build up the CO, could describe a low or restricted burner airflow. Some bugs can easily disrupt this process.

Any bug residue in the burner's section can mess things up as well. Before you take the trailer to a professional, it would be worth your time to make a cursory inspection of the burner section. It may not need any repairs, only some specific cleaning.

When you're done cleaning, the exhaust vent should have a good air flow from the fan, and the flame a bright, sharply defined, blue. Turn the furnace on high for a while and see if the detector emits an alarm. Don't stay in the living space while you're doing the testing.

Also make sure the CO decector is up to date and its battery is new. Both are time limited and eventually have to be replaced.

"Since gas leaks are not something to mess around with, I wouldn't hesitate to take it to a professional, especially if you are smelling gas. The money spent may save your life. IMO"

Very wise advice