The principle of "room sealing" in the context of gas appliances is not defined as "air tight" because air tightness is impossible to achieve, so there is a formula that involves pressure differences, burner power, and other factors that defines the maximum permitted levels of flue product transmission/ leakage to a habitation volume.
The fridge vents on the outside of the caravan are not part of the statutory habitation ventilation. If the fridge were fitted in accordance with its installation instructions, there would be no air exchange between the fridge vents and the habitation space. However most of us know that the caravan manufacturers have got away with poor room sealing of the fridges for many years, principally because the the fridges flue is directed outside and any blow back into the fridges vents would be diluted even further, enough to render a very low risk to occupants. I'm not defending the caravan manufacturers practice, as they have had more than enough experience and time to comply with the fridge installation instructions.
Space heaters are on different scale of burner size and therefore risks are greater should the room sealing fail to meet the required standard.
Due to the design of space heaters, the room sealing to the required standard is much easier to achieve, and is usually managed successfully and safely.