OK just to close this out, the CO detector is quite sensitive to battery voltage. Opened up the compartment containing the house battery, checked the date code, and I'm quite amazed that the ORIGINAL house battery was still in there holding a charge.
Went to a boat shop today while waiting for Discount Tire to stick some new tires on the SMB, and discussed options for deep cycle batteries. A 7 year old battery might hold a charge today, while the temperature is in the 60's, but its days are numbered. When it comes to batteries, I like to err on the side of replacing them too soon.
The guy at the boat shop said that he couldn't believe that this battery was still ticking. I replaced it with an interstate battery brand deep cycle unit, it was a perfect and exact fit in the custom fiberglass box that SMB austin had built for it. I decided against the AGM battery due to the likelihood that I'd need to also replace the charger. Just not in the cards right now.
So we'll feel more secure with a new house battery in the back, and hopefully this will let us sleep all night without the CO detector bitching at us.
Tent camping for now. Dreaming up a new 4x4 SMB. Gas or Diesel?