Be sure not to confuse CO
2 (carbon dioxide) with CO (carbon monoxide). Seems to be happening in several threads lately.
CO
2 is a byproduct of human respiration as well as combustion and occurs naturally in the atmosphere. It is only dangerous in high concentrations as it displaces the
oxygen. As you say, keeping a window or vent open slightly is enough to keep it from building up excessively when cooking or even sleeping (especially with four of us in such a tiny space).
CO is a highly toxic byproduct of combustion sometimes formed in small quantities along with the normal CO
2 and H
2O. Since it can be fatal in low concentrations before you are even aware of its presence, a working carbon monoxide detector is a critical safety device, both in the trailer and at home.