I've run into similar issues. Dealer changed out the propane regulator as they noted the water heater not burning clean.
Even since then the CO levels bounce up and down when we use the water heater, furnace and the fridge is on propane.
I've not tracked down any leaks from the exhaust of the appliances to the cabin but I suspect that fumes may get sucked in at times.
We always ensure good ventilation using the fantastic fans to keep the air moving around.
My thread on our issue
Carbon Monoxide leak from water heater !!