Carbon monoxide is a by product of combustion in the furnace burner chamber. It is supposed to be forced out of the burner chamber by the combustion blower as the propane burns. As a furnace cycles on and off the burner chamber expands and contracts. While the metal is designed to do this it has a finite life span. Add to that a high humidity environment and lack of use and you get rust. Rust excelerates the metal deterioration. If allowed to continue the burner chamber will fail at a joint or due to rust through. Once it fails the combustion blower will tend to force the combustion gases out through the hole because it's easier that forcing it out the exhaust. It does not take a whole lot of CO2 to kill. That is why Suburban and other furnace manufacturers are so cautious.

Why doesn't this come up with household units you ask? Good question. Step back and take a look at the specifications for an RV furnace and your household unit. That little bitty box is turning out fully 1/3 the btu's of that great big 5-6' tall thing in your house. The household unit moves massive amounts of air through big ducts while the RV unit crams air through 4" round ducting.

Charlie