Explain vehicle / equipment top air conditions not on RVs.

A friend has a bus company, they have several small shuttle buses (25 passenger?) built on a Ford F550 chassis. He says that it keeps up just fine on the hottest of days, and bear in mind, 25 adults would be the equivalent of 2,500 watts of electric heat running over and above just cooling the space.

The rear roof-mounted a/c runs off a second engine run compressor. IIRC the largest Ford a/c compressor setup runs 3.5 pounds of refrigerant so it would have a theoretical capacity of nearly 5 tons.