The amount of ventilation a propane heater needs is dependent on the heat output of the propane heater within a given amount of space. The more propane is burnt within a given amount of space the more ventilation is required within that space. There is no exact science as to how much ventilation you will need because the air will escape at higher/lower rates depending on how high the vent is within the RV and how obstructed it is. Air may also escape at different rates depending on how well insulated your RV is. Overdoing the ventilation may make you susceptible to leaks, cold air, and moisture, effectively nullifying all the work your propane heater has done. All in all, it is most important to have multiple sensors which detect the amount of oxygen in the air, the presence of Carbon Monoxide, and the presence of Liquid Propane (LP) gas in the van. In this way, you can increase your ventilation if you are detecting the presence of dangerous gases.