Over 24 feet and you need 2 units. It is hard to believe they would sell a 31 foot unit in Florida with one AC. The best a properly sized AC unit will do is 18-20 degrees. If it is 100 out then it will only get you down to 80. Since an RV is limited to the size you must add a second AC. RVs that are 38 - 42 need 3 AC units.