According to the domestic gas appliance regulations, gas appliances should not be installed within a specified area of an opening window to reduce the possibility of exhaust fumes from the appliance entering the living space. That make a great deal of sense especially as most domestic appliances can consume several kW of gas and produce copious amounts of flue products.
Technically the same provision applies to caravans and motor homes and it is still preferable to keep the risk as small as possible, but there are mitigating circumstances which teh caravan manufacturers tend to rely on.
Firstly the gas consumption of the appliance that have wall mounted flues, are typically less than For example a fridge typically is about 120W, and a water heater is less than 1200W.
In the case of the fridge where may fridges do burn continuously the volume of exhaust gasses is considered to be small and any window would still also take in substantially more fresh air as well diluting the exhaust to a very low level.
For the water heater, the burner does not run continuously so the averaged volume of flue products will be much lower than the burners rating . Typically when a the appliance is turned on and up to temperature the burner fires for less than 25% of the to maintain the water temperature. this means on flue gasses are more like having a 300W burner. Like the fridge this is accepted as a small combustion system, and whilst not ideal to have directly under an opening window the mixing with fresh air, it should only pose a slightly raised risk.
The same argument is put forward as to why fridges are rarely well room sealed around the furniture front frame.
I suspect it would only take a serious incident with one of these types of installation to make caravan manufacturers have to adopt the full domestic approach to choosing installation sites.