I’m not sure what brand of CO detectors you have, but it may be possible they are sensitive to the boiling of the electrolyte that is probably taking place at the batteries. It's also possible the detector has become contaminated or is at the end of its effective lifespan. Check the "Replace By" date tag or label on the back of the detector or in the literature provided. Most detectors will need replacing at about the seven year mark, but check your date code to verify.
Without having the benefit of running some tests and taking some measurements, I’d hazard a guess that the AC-DC converter is overcharging the battery bank every time you are plugged into shore power. Be sure to keep a close eye on the electrolyte level in each battery in that bank until you rectify the problem. It can literally boil the water right out of the electrolyte, produce heat and off-gas the hydrogen and sulfuric acid. This is the odor, I believe, you are smelling.
In my opinion, many stock charging converters are not sophisticated enough or engineered with circuitry designed for the complete and proper charging sequences necessary for optimal charging of commercial grade deep cycle batteries. Many have a propensity to overcharge the battery bank. I’ve always suggested investing in a dedicated three-step battery charger and allowing the converter to convert and have the battery charger, charge. Multi-stage, processor-controlled chargers can be user-set to accommodate any type of battery so it knows how to charge, when to charge, and at what current and voltage so that overcharging does not happen.
But be sure to verify the integrity of the CO detectors! Never compromise the safety factor! You'll want to update your detectors if they need replacing. They sounded off for some reason!