In some cases the air conditioner you have may do a good job keeping the interior of your RV cool. Even so you may want a second unit for a specific part of the RV like the bedroom. In addition, old, box-like motorhome air conditioning units tend to be a drag on RV mileage and so upgrading to a sleek modern design can save you a lot of money at the pump. And in some cases the rest of the RV simply outlasts the original camper air conditioner which then needs to be replaced.