if the CO alarm is only sounding off with the water heater on - it sounds like you may have an issue with the water heater leaking carbon monoxide. If your water heater does not have an electric heat option, I would turn off the water heater altogether until you can get it looked at or fixed. You can heat water on the stove - so the lack of the water heater should not ruin your trip. But CO leaking into your camper will.
HTT: "EscapeII" 2016 Shamrock 23WS (current)
PUP: "Escape" 2010 Rockwood HW 277 (gone)
TV: "Bruce" 2011 Dodge Durango V8 Hemi Crew
Just us gals (me, Sis and our daughters)
We spend alot of money to go sit in the woods