The first time I used the hot water heater, the alarm went off. So I shut the hot water heater down. There happened to be another Rockwood A-frame at Lathrop State Park (Southern Colorado). We greeted each other, and I mentioned what had happened. He said his had done the same thing, and he was told to reset and try the water heater again. The alarm never happened again.
I went back to my A122, started the hot water heater again with no issues. And starting the hot water heater has never caused the alarm again.
I would first check the hot water heater vents and air flow, as has been suggested.
The A-Frame CO detector is a combination propane/CO/battery low voltage detector. Make sure you are getting an alarm for CO, and not propane or low battery.
Any CO alarm has a limited lifespan. The newest ones for home use claim 7 years with a non-replaceable battery. The model installed in my A-frame claims 5 years from date of manufacture (not installation) - whenever that was. I assume the clock started sometime in 2013, since my A-frame was made in March 2014. Others have reported problems with the propane/CO alarm well before the 5 year date, so not all units will make the 5 year life.
If nothing found with the water heater, and if your A-frame is 2013 or newer, I would think about getting a home battery-powered CO unit ($25 - $30), and using it as a side-by-side comparison. Or just replace the whole alarm (not sure of price).
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time