Gasoline motors generate high levels of CO.
Against common perception, propane motors generate similar, if not higher, levels of CO than a gas motor. (theoretically, a properly tuned propane motor will produce 40% less CO than a gas motor, but I have rarely seen one stay in tune very long). The motors develop leaks in their flow diaphragms and start running a little rich, pouring out massive amounts of CO. We battle this constantly in our shop with forklifts and regularly hear the CO detectors screaming. LPG (Propane) is used in industrial applications primarily because 1) it doesn't produce many oxides of nitrogen when burned 2) it is not a fuel that must be "handled" like liquid diesel and liquid gasoline, which OSHA virtually forbids the handling of liquid flammables in all but an approved fueling area which most small mfrs do not have.
Diesel put out 15% of the CO as a gas motor. In addition, diesel also puts out some stuff in the exhaust that will make you vomit if exposed to high levels of it. A person will wake up puking if a diesel genset cracks a manifold and fills the boat up with exhaust. This is far better than never waking up. (plus you get DOUBLE the runtime on a given fuel tank over a gas genset).
I've hung my CO detector 12" to one side of my diesel genset exhaust, and 2" above water level. Boat was docked in its slip, no wind. Ran for 30 minutes. Detector showed a 6. OSHA will let you sniff a 6 for eight hours.
My Nissan propane towmotor will fill my entire loading area to a 40 in ten minutes. My Caterpillar towmotor will show a 130 in ten minutes. Any reading over 80 results in an instant alarm, while lower readings are time-weighted. The 40 reading will alarm in about twenty minutes.
My Ford 1/2 ton can run for 30 minutes and get an 8, but that is fuel injection and catalytic converters and no-load idling. My SL500 Mercedes can run for 30 minutes and I still read a 0. My Cat powered topkick will show a 4 after 30 minutes.
Just food for thought.