In an American motorhome there are typically two types of air-conditioning systems.
The dash air-con is the system by which the engine will drive the gas compressor within the air-con unit. This type of system naturally looses gas each year, so in order to keep the unit working efficiently gas will need to be added. The gas pressures are checked on the vehicle service. On the older vehicles a gas known as R12 was used for air-conditioning, which can no longer be used. LAS use a substitute gas instead which is a direct drop in, and does not need the air dryer or air compressor oil changing.
The other type of air-con is the roof air-con, which (you guessed it) is mounted on the roof and used to keep the interior of the motorhome cool when the vehicle is parked up. Instead of the engine acting as the pump, roof units use electricity from a land line or generator. The roof units use sealed compressors with no external moving parts. The compressors themselves comprise of diaphragms which are used to increase the gas pressure whilst preventing any gas from escaping. Asides from external pipe damage there should be no need to add any gas to the system. In the case of pipe failure, LAS have the adaptors and gas necessary to refill the units.
On a service the units are tested to make sure they are running efficiently and that the condensers are free from dirt.