There is no definitive answer to this question as it depends on a number of factors, such as the size of your RV, the type of insulation you have, and the climate you are in. However, a good rule of thumb is that you should have at least one CFM (cubic feet per minute) of airflow for every square foot of floor space in your RV. So, if your RV is 30 feet long and 10 feet wide, you would need a fan with a minimum airflow of 300 CFM.