Carbon MONoxide, CO detector. There is very little hazard from moderate amounts of CO2, Carbon Dioxide. CO2 is in our carbonated drinks. I'm sorry to make this correction, but I have to hear our fire dispatchers make this mistake time and time again. CO is A KILLER! Whether your TT, 5'r, or coach came with a CO detector, MAKE SURE YOU HAVE AN OPERATING ONE! The hot water heater can produce enough CO to kill you if not exhausting correctly, and it can have been jarred just a little bit. CO is colorless, oderless and is the same weight as air so it doesn't matter where you put the CO detector. Just make sure it is rated for an RV, it will say it on the box. They are sensitive electronics devices and some can be damaged by jarring. They also have a life. I think that the sensors are going about 8 years, REPLACE EVERY 5 YEARS. I lost a firefighter buddy I went to high school with in a fire, and it was found that he had CO poisoning from home. His hot water heater in his house vent was not correct. The fire he was fighting was enough to put him over the lethal level. His death saved his fiances life by allowing her to get treatment in time. CO affects the young and the elderly first and severe! Please make sure you CO detectors and your smoke detectors are in working order and CHANGE THE BATTERIES at least every spring. If you're a full timer, at the time change (this coming weekend). That is my safety topic of the day!