Personally, I would like to have it near my head where I am sleeping, as I feel that that is when I am most vulnerable to a Carbon Monoxide build up.
It turns out that there are also rules for where NOT to put a CO detector (at least in a house).
"The following locations can either create a false alarm or avoid your
detector from properly identifying the CO levels in your home:
* In close proximity to any fuel-burning appliance.
* In excessively humid areas such as your bathroom.
* In direct sunlight.
* Near any sources of blowing air such as a fan, vent or open window."
As you can see from that, it can be tricky finding a good place in something as small as an RV.