Keeping in mind that most people are sitting, laying or standing in a caravan, they need little in the way of air, in fact the average person uses 7 liters of air a minute sitting or doing light chores. Which is very little. And while we all breath and sweat not much water vapor is produced. In my opinion it would be easy for the manufacturers to provide adjustable ventilation, with guidance for the user. If you use an electric kettle or hob then most of the current ventilation is excessive.
At a time when we are all encouraged to use less energy, the manufacturers are missing an important trick.
In days gone by caravan manufacturers did not provide fixed ventilation, and there were a significant number of cases where injury due to reduced oxygen ensued. Users had the opportunity to use variable ventilations but there were enough instances where serious injury or even death occured to cause the Government to make fixed ventilation mandatory.
Air normally contains 20% oxygen, humans begins to suffer significant problems if the Oxygen percentage drops below 19.5%. we breathe in air with 20% O2 but we breathe out only about 15%. O2
If we were in a closed caravan without ventilation, based on your figures we would consume at least 10,080L of air per day. That is more than half the internal volume of a 5M long caravan! The reduction in the percentage of O2 in the caravan would be in the region of 2.5 to 3.5%! And that's for just 1person!
Add in the oxygen used by the cooker hob and oven, plus the raised Carbon monoxide levels, and you have a seriously deteriorating situation. it would be even worse if there are more people or animals, or it was a smaller caravan.
As for moisture from breathing, an adult can liberate 50 to 100mL of water over night simply by breathing. The use of a kettle or pan will add to the airborne water vapour load, And whenever you burn LPG, roughly for every gm gas consumed you will liberate 1 GM of water vapour in the product of combustion. Obviously room sealed appliances will conduct all the water vapour outside, but none roomseald appliances like hobs & ovens release all their flue products into the living rooms. Ok